We have two roundabouts and I really like them. My girls are in the 5th percentile for weight and the 50th percentile for height though, so they should be fine in them for a long time. In fact, they still have plenty room in them rear-facing at almost two. I'd definitely recommend them. BRU doesn't have the prettiest pattern for them in the store, but they don't show dirt at all- which is a huge plus!
Decathlon here. Bought it b/c it looked cushier than the marathon and it has the crotch strap that moves to three different positions according to the child's size. It rear faces to 33lb and 5 point harnesses to 65lb. It also has an infant pillow and toddler headrest with it. We got them at buy buy baby with 20% off BRU and Bed Bath and beyond coupons.
Re: If you have the Britax car seat...