FWIW,
I have 2 boys (20 months and 3 3/4 born on the same day 2 years apart! - not sure if that's nearly twins) so have run and am still running through the whole gamut of car seats.
We started with a Graco system - rear facing seat, just plug it into the base which remains permanently in place for the 1st year. We bought an extra base for the second car which was a boon - NO fiddling with straps etc. and baby isn't disturbed when you put them in or take them out of the car. This system served us well in the early days in our 900 convertible and a Freelander. Oh, yes, I'm 6'0" and there was no problem with leg-room for me, or seatbelt length (very little is needed to attach the base.) Being a 'system' it meant that pram-car-pram transfers were very easy - well, as easy as they can be!
When we went on to forward facing seats, we bought a couple of Maxi Cosi (Priori) seats which fitted the Saab perfectly but not the Freelander (the attachment points are a long way forward than normal and only the Britax Renaissance fitted correctly - which happened to cost twice as much!)
When we got the Volvo (replacement for the Freelander) we bought their own rear-facing car seat at great expense but already James has outgrown it - not in physical terms - he just doesn't like facing backwards anymore - so that was a very expensive (£250) purchase considering how little time he has spent in it. If Saabs own seat is engineered anything like the Volvo then the seat in front of it will have to be fully forward, leaving you with very little legroom!
The 2 maxi cosi seats are in constant use in my '04 93 convertible and prove to be very practical - they can be reclined to any position without having to adjust any straps etc. Thomas is just about outgrowing his seat (he's been using the built-in booster seat in the Volvo since we got the car) so I will be in the market again for a booster seat for the Saab shortly.
In short, don't go for looks, go for practicality. The Graco system looked and felt cheap (although sturdy enough - it was 'clunky') but it worked very well, M&P systems look cool, but cost an arm and a leg. Anything that requires the fastening and unfastening of the seat belt everytime to want to load/offload your offspring will have you cursing by the time you move on to a fixed forward facing seat.
Just my 2 cents,
ATB,
Phil
PS For Simon K - Actually I've found the convertible to be very practical for a couple of kids and associated paraphenalia. Particularly in this fine weather we're having now - as loading and unloading can be done over the side of the car - whilst others are struggling to climb in and out of the back.....Enjoy!