"most" kids do go into a booster seat after they hit 40 lbs. However, there has been quite a bit of controversy lately over the fact that 5 pt harnesses are safer and 5 pt harness seats that go over 40 lbs are rare.
I have a Marathon and I think it is one of the best seats around because 1) it can be used rear facing up to a higher weight limit than most convertible seats (ie you can keep it rear facing after the child hits 1yr/20 lbs, which is much safer)
2) it fits more tightly in my car than any other seat ever has....it is seriously rock solid, and it is so easy to make sure you did it right
3) it goes up to 65 lbs, so you don't have to buy another seat for quite a long time.
I have a Triumph as well in my husband's car, and it is a good seat too, but I personally like the Marathon better and my son has always seemed more comfortable in it as well. I don't mind spending the extra money to ensure his safety, plus if I bought other seats I would have spent more money to keep upgrading seats anyway. I am most likely buying another one for my next child and may not even use a traditional infant seat if the weight limit goes that low.