Pros: Stable, quick, comfortable, fun
Cons: Portability, and your hands aren't free
The kickbike is an ideal mix of rolling and walking/running. It is similar in concept to those little razor scooters you see kids on, but has much larger wheels which gives it far greater stability and smoothness. The front tire is 700c (a common bicycle tire) and the rear wheel is 16". The wheels are pneumatic (air filled) like bike tires rather than hard wheels like on skates and other small scooters. Between is a thin platform you stand on, and you kick to propel the kickbike same as any other type of kick scooter.
Overall, the kickbike's appearance is rather bicycle-like, at least from the front. And since for some reason this is the first thing people always ask: yes, you do alternate which leg you kick with while riding :) . I switch legs every 3-10 kicks depending on conditions.
Below I'll compare the riding experience in more detail to a regular bicycle and to inline skates, but I think overall the key feature is it's simplicity. It's a very easy and care-free riding experience.
An ordinary bicycle can be great out on the open road, but isn't as much fun at slow speeds with frequent stops and starts. The kickbike is very strong in that area. It is easy to control all the way down to sub-walking speeds. And unlike a bike, it's trivial to transition back and forth between rolling and walking. That gives a much greater sense of freedom than any other wheeled vehicle I've used. For me this makes sight seeing more enjoyable from a kickbike that from a regular bike.
One measure of the vehicle's flexibility is how you feel at an intersection where you need to walk over and push the cross walk button. On a regular bike that task can be surprisingly awkward, and then the light invariably changes too soon before you're back on the bike. On a kickbike the instant transition between rolling and walking and the tight maneuverability make the task trivial.
The kickbike also has ache-and-pain related advantages. I'm sure you can deduce this advantage for yourself, but there's no seat pain on a kickbike. And it's very knee friendly. On a regular bike my knees start to hurt within an hour, but not on a kickbike. I assume that's because I'm not locked into a rigidly constrained motion.
The cost for the above benefits vs a regular bike is of course speed. I would say a regular bike is something like 50% faster overall. I spend most of my time around 12-14mph, with 10mph being a relaxed stroll and 18mph getting close to a sprint. I read somewhere that the world record marathon time (26 miles) is 1h 20m on a kickbike.
I often take the kickbike out on social bicycle events, and it mixes well with the bikes and is easy to keep up at those speeds.
Kickbiking also compares favorably to inline skating, mainly in the ability to handle rough terrain. Little rocks and potholes won't trip up a kickbike the way they will inline skates. It's a vast improvement for comfort and stability over rough terrain, and I feel much less tired after going longer distances. I usually kickbike at least 15 miles, and at the end of that distance, I would have no hot-spots, unlike on skates where it's a relief to get my skates off to let my feet relax.
It does have a few disadvantages vs inline skating: It requires both hands on the handlebars, where in skating both hands are free to drink water from a bottle etc. It's much less portable than inline skates. And I can't hop on and off curbs on the kickbike like I can on skates. And of course skates aren't susceptible to flat tires. There is something satisfying about crunching over broken glass with impunity on skates. As far as speed, if you skate well then skates are faster, if not then the kickbike is faster.
As far as the type of exercise, I find kickbiking is primarily cardio, where skating and biking both make more muscular use of the legs.
Of the kickbike's minor disadvantages, the only one that routinely gets in my way is its portability. The wheels do have quick release that works fine, but that's not enough to make it fit easily in my car. If the rear seat of your car folds down it'll be okay, but mine doesn't so it's quite unwieldy to make it fit.
I've never tried fitting it on a car-rack, so I don't know how much clever jerry rigging it would take to make that work.
But overall the comfort, ergonomics, freedom, and all-speed stability is a big enough win that it's my favorite form of human powered transportation.
The only context in which I choose a regular bike now is when speed matters. For example commuting to work, for that I do use a regular bike. But for recreation, it's all kickbike.
As far as build quality I've had no problems. You will scrape the bottom on curbs from time to time, and it is appropriately reinforced to handle this. I've found the platform you stand on to be of ideal size and texture: solid traction but not too hard to pivot the toe or heel out when switching feet (you're "supposed" to pivot on your heel bringing your toe out when switching feet, but I do it both ways). Mine has around 12-13" usable length to stand on and is 4" wide. I've heard the 2007 model is a little longer to accommodate larger feet. Although this may sound small, it's actually ideal for the vehicle. I never stand idly with both feet resting fully on the platform anyway, and the one foot that is on the platform should be centered, not off to the side, hence the 4" width.
The only other related vehicle I can compare to is a xootr mg, and there is no comparison. I don't mean to be too negative on the xootr, but it is comparatively unstable and twitchy and you can't put any power into your kicks. I also find the balance and usability of the kickbike's thin platform much nicer than the xootr, which is wide enough that you're frequently standing off center which causes balance problems and makes it harder to kick.
I still believe it's possible to make a more portable version of a kickbike with only moderate performance and handling tradeoffs, but I'm not aware of anything better than a xootr yet in that vein.
The bike does come with a small kickstand which works reasonably well, it's just a little too small so the bike can fall over too easily. I've seen pictures of the 2007 model and it looks like they've improved this feature although I can't comment from experience. I will say that the old kickstand design (where a little 5" stand unfolds from under the platform you stand on) is really cute though.
It also comes with fenders, although I took them off which helps with portability. By default the millennium doesn't come with a stem riser, and I believe that's still true of the 2007 model. At 5'10" I've found the bars to be at a comfortable height. Some people add a stem riser though.
I've heard that tire widths have varied a bit from year to year (and I'm not even positive which year model I have, although I think it's 2001) but mine has 700/25 on the front and a 1+3/8" 16" (349) on the back. This may be my inline skating background talking again, but when I hear other people wanting even wider tires than this for an even smoother ride, I can't imagine why. From my perspective this is already incredibly plush as is. On gravel or dirt I would indeed want wider tires, but on pavement of any quality I've been delighted with the ride on the thin tires.