We umm'ed and ahhh'd over buying one of these for our Graco car seat.
Eventually we did, and are so glad.
It is really easy to fit, and makes transferring the little one from the car to the buggy much easier and quicker.
In our car (Vectra Estate) it is ideal, with loads of room, however, in my Mum's car (Peugeot 206) there isn't a lot of room above the seat to get it out, once you have the base then the seat.
You do not need ISOFix for this, it simply straps in with a seatbelt, then the leg sits on the floor. Can work in pretty much any car, as long as you have a full seatbelt, and of course, no airbag if it is in the front.