We have one too, a four seater in a Volvo xc90. Got it when dc3 arrived just over two years ago. DC's now 2,3,6 yrs old. It is a fantastic invention, although it does lack the refinement and I suppose, luxury of more upholstered, tweaked car seats that we are used to seeing.
I would echo pgb8000's comments. I was never fond of the infant carrier either, it doesn't look as cosy and comfy as a maxi cosi, baby does seem more upright (and ds2 was/is tiny...4lbs at birth) but it didn't seem to bother him. I found it really tricky to get it fitted back into the main seat and very heavy too, so never really took it out. If I knew I would want to move the baby seat I would use our old maxicosi in front seat. Tbh i was very glad to be able to stop using the minimac.
The other DC's also seem pretty happy in their seats and harnesses, even on three trips to southern France and back. Also handy having the extra seat for taking a friend home from school etc. We originally asked for the 3 seater but it looked slightly ridiculous in the Volvo, with spare useless seat hanging either side, so we went for the 4. We haven't taken it out of the car since we bought it as it is so heavy (29kg iirc?) and cumbersome. This compromises our admittedly already large boot but is occasionally (eg at Ikea
) inconvenient. Also less space to comfortably fit extra adults - the two fold down seats in the back of the Volvo aren't really for adults.
As said by pgb, the multimac was definitely the answer for us, but once DD can sit in a booster and ds2 in an older child seat - maybe 3/4 years - we will probably get rid of it. I can actually imagine over 10's being comfortable in it.
Am in N London if anyone wants a look and has a fumigation suit doesn't mind a messy car 