We've got one (the series 2). The main issues are that back doors don't open that wide and the back seats are high. Saying that, we've not had any problems so far with get the Maxi-cosi cabriofix in and out, and we have the iso-fix base, which was easy to fit.
However, at 9 months I am starting to struggle with putting A in. Now A is over 20lbs he's quite heavy to lift up onto the base if he is already in the car seat, and because of the height of the back seats, it is difficult to lift him into the car seat if it is fixed in the car - I have to manoeuvre him over the side of the seat, which involves stretching forward and lifting him up. It's not impossible (I do it every day!) but it is definitely easier to get him into the car seat in the Fiesta!
On the plus side, the boot space is fab! Easily fits the iCandy and our weekly shop! When he was little we'd lift the car seat out and put it in the boot to take him out of it and put him in the pram, which was handy as it kept him mostly dry.
We've actually just ordered the Britax Dualfix as we wanted to keep him rear-facing and that one rotates for easier access. It's being delivered next week so I can let you know if it fits (it doesn't appear on the fit finder so Mothercare are going to try it for us).
Give me a shout if you want to know anything else about it.