We own a ALL4 countryman in my family only it's the SD version. Everybody loves it and apparently it's great to drive (I'm not insured on it, belongs to my mother). She also owns a Mini Cooper S and although it doesn't zip around as much, the Countryman does have a lot of go, especially when she puts her foot down. I will answer your questions in order to make it easy
1) From experience they are quite reliable. Hers is a 61 plate and the only thing we have had to replace are the front brake pads which was about 2 months ago. This was purely because it does the school run in all that stop start traffic and my mum is heavy on the brakes! I will admit brake pads should last longer than this. Apart from that it's been perfect.
2) Personally I find it very comfy. We have spent long journeys in it with four people inside and the back seats are reasonably spacious for a Mini. I have never got out of the car feeling achey. However it's not great for family holidays as trying to put two large suitcases in plus 4 people is a struggle...it still maintains the 'Mini' boot. However with the back seats down it is pretty good as the seats go completely flat.
3) Ours does about 26mpg so its not great on fuel even though its an SD. As I mentioned earlier nothing has gone wrong apart from the front brake pads (£250 approx)
We also own a X5 and I have to say the countryman wins hands down. It's fun to drive and maintains the mini feel. When it's parked next to other SUV's it does look surprisingly smaller as well.
Hope this helps