We have the britax first class si, and my parents have it for cole also and its really good. It reclines fine and it sits up ok too and I've just discovered the button to loosen the harness easily

We've been happy with the seat and its fit in every car no problems including our escort which has funny seatbelts apparently so not many fit. I think including parents cars, mums old car, ours, and various friends the seat has been in 7 different cars all fitting fine (we check on teh website first)
Cole went in his seat at 8 months old because we had to go to a wedding and it was a 5+ hour trip and we figured he'd be happier with more room and it would be better if we could see him and he could see us. It did make the last hour of horrible traffic on the m25 slightly easier as at least I could see him when he was crying and fed up.
We had cole weighed before we got the seat and he was the right weight, the guy at mothercare also said he should be able to sit up on his own for 20 mins before he went in the seat and said the age wasn't as important as the weight. The nasty cow on the tills however tried to lecture us because we were daring to consider putting him in the seat a month early - and we were buying it a month before we intended to use it so he was 7 months old, and she was busy telling us he'd die if we had an accident - despite the fact that he was practically the right weight at 7 months, and he was starting to sit up then and we were clear about the fact that we weren't going to use it for another month. She was a total cow.
As it happens, he was sitting fine when we wanted to use it, and he was quite happy in it and loves looking out of the window and stuff and was just fine. He sleeps comfortably in it, he sits fine in it and i'm totally happy with it. Its not too fiddly to fit and it has instructions on the side which is handy as it means you don't need to dig outt he manual if you want to move it.
hth