Get a Britax Marathon instead of the Roundabout. You can use the Marathon much longer than the Roundabout. Do a side by side comparison if you wish. I love how the EPS foam on the Marathon really hugs and supports the body.
Their booklet instructions are great. I have not had any problems with ease of use. My husband has a Sunshine Radian 80 folding car seat in his car for my 4 yr old and it's OK, but again, it does not hug and support her body like the Marathon does.
I had a Roundabout for my son (born in 2000). My daughter was born in 2004 and I got a Marathon for her. She is 4 now and still rides in the Marathon comfortably and can use it for many more months. If she were in a RA, I would have had to purchase another one by now.
My son rode in a Britax Husky (renamed the Regent) until he was 8 years old (5-pt-harness). Also another great seat.
This is a discussion board with parents who are car safety techs answering your questions. It's a great resource.
http://www.car-seat.org/
Do you know that experts are recommending rear-facing for longer than the MINIMUM of 20lbs and 1 year? Just FYI.... you can find all the info and data on why on that site. Notice what happens in an accident when the child is rear-facing (almost NO movement or trauma to the child.) You cannot say the same if they are FF. Some parents are keeping their kids RF up to 3-4 years (like in Europe). Again, just so you know... you can read more on that website - car-seat.org.
http://www.youtube.com/watch?v=K62Ea8Fs4ng&feature=re...
And, the above poster is right about the Radian 80. If you need to fit 3 across, get that. I did not, so I'm fine with the Britax.