Pros: Easy to buckle, substantial, wipe clean
Cons: Good, nice looking seat, but a tad expensive.
This review is for the Decathalon in Onyx Black. Please note the photo displayed with this listing is not correct. We bought this seat after our daughter outgrew her infant seat at 4 months.
We had a Combi Savvy travel system before, and one of the main complaints we had from that was the difficulty with the belt buckle. It required a certain amount of dexterity to get it to close and latch, which was hard to do with a screaming, squirmy kid (and if she was asleep, you hated to fiddle with it and wake her up). So when we got another seat, we were looking for something else.
We first got a Combi Avatar only because they can be found CHEAP on Ebay, but decided that the grandparents could use it as the "one in awhile seat." We tried a couple of other seats out and settled on the Britax Decathalon based on reviews we'd heard. We considered the Marathon and Roundabout seats, which are both highly rated and cheaper, but didn't pick them because the fit and side impact protection on the headrest for the Decathalon was much better. The Boulevard, a tad more expensive, has a rigid inset in the headrest area to protect from side impacts. However, it is not removable as it is on the Decathalon.
The belt buckle on all the Britax seats is certainly easier to use. The Combi system required the tangs on the two shoulder straps to click together, before being inserted into the center buckle. It looked cool (nice aesthetic) but was a pain, especially for my wife, whose hands are smaller than mine. The Britax buckle, conversely, allows you to buckle each strap independently.
It was fairly easy to install using the rear shoulder belt (we don't have LATCH in our car), but it required a bunch of trial and error to get the angle right so baby could sleep comfortably in the rear facing position. This seat does not have a level gauge, but then I don't know if this is common on convertible seats (most infant seats have one). I now have about 3 towels rolled up under it to get it reclined enough so that her head doesn't slouch forward when she conks off. The base is pretty small, which is nice, except it was harder to "shim" with the small footprint. One benefit of this seat is that when installed in the rear position, you can still get your hand between the seat back and the front of the carseat without loosening the shoulder belt. Our Combi convertible fit so tightly against the seat that you had to disconnect it to adjust the lower strap.
The seat looks big, but somehow it fits. Our car is a Passat wagon, and it fits fine with a little bit of adjustment to the front seats (down and slightly forward). I am average 5'10" and this works fine for me, though I tend to like my seat closer to the pedals.
Lastly, the plastic frame is lined with impact styrofoam, similar to a motorcyle helmet. To my untrained eye, the protection this seat offers from side impacts is superior (looking, at least) to other brands on the market. The headrest also has side impact protection, which we like since the car seems to have airbags coming out from all directions.
Onyx is a nice color because the seating area of the cover is nylon, and thus is wipe clean and doesn't show dirt, plus it matches with our black leather in the car. Other colors are more exuberant, but as a whole I wish car seat manufacturers would tone it down a bit.
The cons: big and pretty dang expensive. When we go out for a drive with friends, the small people have to sit in back. We have the seat in the center position, which works best with the belts, but it is wide enough that people on each side have a hard time finding the buckles for their seats. It is also tall enough that you might need a bit of practice getting an infant in and out of the seat without bonking her head on the roof of the car. A lower profile would be great (or else, a bigger car, I guess). Also, for some reason, when driving the car this height also makes it seem pretty obtrusive. We often turn from the front seat to give her a binky (or a finger) when she is upset, but the size of the seat makes this really hard if you are under 6'5" and are not double jointed. We went for it despite cost assuming we'll be able to use it for a long time before moving to a booster. If this weren't the case, we'd probably pick something else. For our rear seat, the angle of the seat makes the recline of the Decathalon totally inadequate in the rear-facing position. Hence the aforementioned towels.
I am of the opinion that there really isn't a convenient convertible seat (an oxymoron) but this one is pretty easy to use. They aren't very easy to find discounted, and you probably don't want order one sight unseen anyway--you'll want to put it in your car first, especially if you have a smallish vehicle.