I had my reservations at first before i got my x1, mainly about size also.
Like will the back seat be comfortable for 3 rather large passengers and will the boot be able to accomodate 4 golf bags. but, truth be told half the time our cars are only carrying ourselves and if you do have to pick up more people it's wouldnt be a regular occurence.
I personally find the quality of the cabin pretty good, I don't find the plastics cheap, other than the gear lever and the storage under the arm rest is pathetic. If you compare the X1 with the vw Tiguan, the x1's interior is whole lot better.
If you option it out i think it would be fantastic car!
you have to get the sunroof, sat-nav, memory seats, auto headlights, bi-xenons, auto wipers, front and rear pdc(love it) and the x-line pack makes it look more sporty.
& don't get the 18i. I feel it's a little underpowered for such a heavy body.