We moved DS, 3.5, up from Roundabout to the Britax Marathon about 8 months ago and we really like it. DS is a pretty big kid, 41" and 40 lbs. and I think the seat will still fit him well for another couple of years. In fact we just bought another one for DD, 14 mos., last week and the first time I put her in it she gave a big ear to ear grin and started swinging her feet--of course she may have just been tickled about facing forward for the first time.
One of the things that I really like about this carseat is that it is so easy to take apart to clean--DS gets carsick, so for me, this was an important feature. Also, even after many washings, it still looks new (we have the arctic cat cover).