I was really surprised when I started looking for seats for this baby (#4). Things have changed drastically. I never thought I'd look beyond Britax for a convertible, and Graco for my older kids, but my Safety First Onboard 35 arrived yesterday, and for convertibles my first choice is probably a First Years True Fit.
For some kind of perspective, when my 11 year old was born, I did the research and looked for the best seats available. What I learned was that most infant seats only had a 3 point harness, but I should look for a five point harness (which was very unusual). I ended up with a 3-point harness infant seat lent from a friend (my next daughter had a Fisher Price Safe Embrace Infant Seat -- 5-point harness, but also sold without a base. This review was from about the time she was born http://www.epinions.com/content_69917904516
-- for CPST's the fact that it rear faced to TWENTY TWO pounds was a selling point! I think it was the highest weight limit available on an infant seat.) It had no base -- not all infant seats did at the time. Also, convertible seats were available in overhead shield, T-shield, and 5 point harness models, and making sure I got a 5-point harness model was important. The Britax Roundabout was unusual because it rear-faced up to 30 pounds, while many/most seats on the market still only rear-faced to 20 or 22 lbs. I chose a Roundabout because I wanted the best available, and a seat that rearfaced to 30 lbs. & had straps that didn't tangle up like crazy when you looked at them WAS the best available.
Basically Britax built its reputation in people's minds by being one of the earliest companies to rear face to 30 lbs. and by having easy to use 5-point harnesses with non-twisting straps. They preserved their position by filling that display space in high-end baby stores with seats covered in cute, trendy fabrics that appealed to parents wanting "the best." They just haven't made substantial improvements since the Marathon, and their prices are still as high as ever.