I have used platypus and the MSR hydration accessory for all of my hikes. But if all you do is carry water and nothing else then these systems can be a bit awkward because they do not fit perfectly inside of backpacks. So I gave in and bought a CamelBak lobo from REI with birthday money. I have been quite pleased with my purchase.
I bought the Lobo because it had the water capacity I wanted and it appeared to have the comfort level I needed. The straps are not as thin as other CamelBak backpacks so comfort is a little higher. This was my number one concern. I was really concerned when I saw a shoulder mesh strap on some models. I was not about to buy this design (but I can see it being an advantage if you need breathability.) This model met my requirements on comfort and water capacity.

Why I got it.
I choose this model because I felt the straps were the most comfortable and the capacity was what I would need for a 4 hour hike in a hot desert or a day long hike in a cooler climate. I also got it because I needed (I mean wanted) a bladder system that did not get in the way of my hiking and scrambling on rocks.

How it works.
You do not need to take the bladder out of the pack to fill it up. Just put the large opening of the bladder under your water source and fill it up. Carefully screw on the large cap and go. To drink you just need to bite down on the bite valve and suck. Since gravity is working for you it does not require much sucking to acquire the water. There is also an on/off valve that needs to be turned on so the water can flow.

How the harness system works.
The shoulder straps are universal and are not sized like backpacking packs (I think they should be.) You have a very long strap at the bottom of the shoulder straps that give you as much length as you want. I would guess that a 5 years old is the smallest that can wear it properly and it looks like it can extend up to someone that wears very large clothing. If you are the size of a Sumo wrestler then you may want to pass (But I doubt you will see very many Sumo wrestlers at your local REI or on the trail.) The loose strap is rolled up and held down by a piece of Velcro and it does not get in the way.
The waist straps are also very adjustable. The maximum waist it can support is 49 inches and you can make it as small as you like.
There is a carrying strap on the top of my backpack for carrying by hand.

How to fill it.
The bladder fits snugly in the top pouch of the backpack. You simply unscrew the top, fit the backpack/bladder underneath the water source, fill it up, and screw back on the cap.

Extra pockets and straps
I originally did not want pockets but they did not have a version like this that did not have any pockets on it so I just gave up and bought this. In retrospect I am glad I have those pockets. There are two pockets. The first on is on the bottom of the pack and it is large enough to hold a couple of small sandwiches and small snacks. I can put my Targus keyboard and palmpilot in this pouch, and still have room left over. The second pouch is the flap that covers the bladder opening. This pocket is big enough to hold two wallets and a couple pairs of keys.
On the outside there is a small loop in the back bottom and an elastic band that can hold a small sweater.
On the left side is a mess pocked designed to hold a pump.

My opinion on the bite valve.
When comparing the bite valves I prefer the MSR bite valve over this one. I think the bite valve is comparable to my Platypus valve (maybe a bit better.)

Does the on/off valve work?
Yes it works, but it is not a reason to buy this backpack. I still look at it as a gimmick and the only reason I would use it is when I transport a full bladder.

My opinion on the comfort level
Other than having the waste strap fitted too high above my waist, I have no problems with this particular backpack. The straps are your basic straps but since you carry very little weight in the pack the straps do not need to be very thick or well designed.
The back is moderately padded. The back is pretty flat and not textures to allow air to pass through. Overall I would give the comfort level an A-.

Which hydration pouch should you get?
If you want a complete system including the backpack then get a CamelBak pack. You will be limited to day hikes and minimal snacks (depending on the pack of course) but it is a comfortable system to use.
If you already have a backpack that you are already in love with then get the platypus hydration system. It is a great system and the newer modifications are well implemented. As far as versatility is concerned, I feel this is the best option for most people.
Only get the MSR bite valve system if you are in love with your MSR Bladder. The valve is a little harder to use but has a higher flow rate then either platypus or CamelBak. I do not recommend this unless you already own the system or you really want the higher flow rate.

Things I do not like
Other than the contamination issue, the biggest problem is the bladder is a little difficult to fill. This is a minor gripe but something they need to improve (and I know you're listening). You have to hold the opening with both hands over the water faucet. I have to tilt it to get it to fill properly. And I cannot fill the bladder to its full capacity. I do not have this complaint with the platypus pouch or the MSR bladder.
I do not want to have to buy the clip and wide mouth bite valve separately. These should be standard on all pouches. I will buy the wide mouth bite valve separately next time I go to REI.
The bladder can get infected too easily (they may have solved that with some brand new bladders that are coming out.
The back is not as breathable as it should be.

Things I like
The cap is connected to the pack so I will have a hard time losing it (just the connection piece is a little flimsy.)
The opening is large enough for ice cubes.
2 Small pockets.
The pack is relatively comfortable.

Who is this for?
-People that go on 2-6 hour day hikes.
-People that are sick of carrying a bulky backpack with their water bladder in it.
-Any one that is still carrying a canteen or water bottles when they go hiking.

Weight and dimensions
Dry weight: 1 lb, 11.5 ounces.
Dimensions: 18’ x 7.5 inches x 3.75 inches.
Diameter of the opening: 3.25 inches.
Storage space: 63 cubic inches (listed at REI.com)

How to clean it
If you immediately drain it and put the pouch in the freezer then you should not have any problems with fungus/bacteria growing. If you leave water in the bladder then you will have fungus grow in the pouch and it will affect the taste. You will not see the fungus but you will taste it (I experienced this first hand.)
One trick is to make a low concentration of water and bleach. About a tablespoon per gallon of water for 15 minutes should do it. Bleach is cheap but will damage your bladder if you leave the bleach mixture in the pouch for a prolonged period of time or if the concentration level is too high. And if not properly washed out then you will have a bleach taste in your water.
If you feel uncomfortable using bleach then go to REI and get their special bladder cleaner and follow the directions. It will be more expensive but safer to use. Several manufactures make cleaning agents including CamelBak.

Accessories
Ice Tubes- This is a deep ice cube tray that makes long ice cubes.
CamelBak cleaning tablets- These are tables that will kill the bacteria that try to live on your plastic. If you do not want to use chlorine bleach then these tablets are a good alternative.
CamelBak cleaning kit- Comes with all the equipment you need to clean out your bladder.
CamelBak hanging rack- Helps your bladder dry out.

Additional note
I have the old version that appears to be phasing out. You can buy both version as of this writing. The newer version has better bungee cords and is lighter. It also can carry more gear (146 cubic inches.) It has a quick disconnect tubing. And probably the most important feature, the water bladder and tubing are made with anti-bacterial material. If I knew this one was coming out I would have waited for it and paid the extra 20 bucks (I got mine on sale.)

Summary:
This is a great hydration pack/system. I am really pleased with its design, comfort, and ease of use. The pack will not hold much else but water so do not expect to use this pack for carrying a lot of gear. It is only designed for day hikes.

Other outdoor products you might be interested in.
MSR hydration system
MSR 10 liter bladder
Coleman electric cooler
Coleman Propane 2-burner stove
Camelbak Rim Runner 2010 version. Very much improved.

Recommended: Yes