I was all set to get a Britax Eclipse as it's got great reviews, looks comfortable, good make and good for small cars. Also a good price - rrp is about £120 but can usually find it on sale at certain times for about £80.
Went to Halfords this week and one of the guys tried it in my car (Peugeot 206 with sports seats - not a great combination for a car seat). He got it as tight as he could (more difficult apparently in a 3 door car as can't get the leverage to pull the belt as much) and pulled on the baby straps and it was so unsturdy and easy to move about. Bit gutted, but really wasn't happy it was safe, which is what it is all about I suppose.
He recommended the Maxi Cosi Priori XP as it has this tensioner thing which means there's a lever which tightens the belt. He put it in and it was fab, hardly moved at all no matter how hard you tried. Also a good brand, good recline, felt comfortable and wasn't too big in my wee car. It's rrp is £150 I think but Mothercare has it on offer for £110. I was really impressed by it - I think if you aren't going to go for an Isofix seat (which can be imcompatible with your car and crazy expensive) it's worth getting one with a seat belt tensioner.