Not an owner yet, but I checked out a X1 the other day. Considering I have yet to have fit more than 1 large (standard check-in size, really) luggage, a 14x5x3 box w/ misc stuff (tire plug kit, torque wrench, small towels, and air pressure gauge), 2 14oz boxing gloves, and 2 Thai pads in the back of my car, I'm also interested in the storage capacity of the X1. Looking at a X1 at a dealer the other day, I came out with the following conclusions/thoughts:
The glovebox is small as others have stated in other threads. You can pretty much fit the owners manuals in it and maybe something small on top. The glovebox is pretty well sized to the width of the little case the manuals all go in but you can probably stack something small on top (iPass/EZpass, small EDC pouch, etc...). Personally, I'd put the owners manuals in the back like I do with my coupe.This would leave plenty of room for what I typically keep in my glove box (small flashlight and a EDC pouch w/ misc stuff like a pen, small multitool, etc...) and I'd have room left over. I don't read instructions anyways.
If you don't have Nav, you get a storage box in the center, top of the dash. The X1 the dealer had out had nav so I didn't get a feel for how much can fit in it.
Door compartments....roomy, but I never put anything in them so I didn't really pay much attention to them.
Center Armrest has some storage in it, but it is not cavernous like a Jeep or large SUV's. It looked like the 2nd/removable cupholder get stored in it, though.
The boot is pretty well sized for this size of a vehicle. It is a little smaller but on par w/ a current gen Subaru Outback but has more capacity than a Impreza\WRX\STi (5-door). The cargo area itself is ~25cu ft when the rear seats are in the upright position. To throw another reference, my MCS (Mini Cooper S) only has ~ 24 cu ft when the rear seats are down
...which is still more than my coupe. With the rear seats down, BMW lists the X1's available cargo space as 56cu ft. The rear seats don't fold down completely flat, though. They sit up at an angle of maybe several degrees, but it's not too bad or worse than other vehicles. You won't fit a 7' Xmas tree in it and be able to close the hatch unless it sticks in between the front seats a little. Underneath the 'floor panel' of the boot, is additional storage that spans the entire length and width of the boot area. It's a few inches tall and can fit a lot of misc stuff. There is also a bin on each side of the boot for storing things. Overall, very useful storage/cargo space. Having the 40/20/40 split folding rear seats seems great, too.
As 2Bims said, without knowing what it is you're planning to haul, it's hard to say. Best way to find out is to see one for yourself. I have no doubt I can haul a full set of 18" wheels/tires in the back without having to stack them on top of each other. I walked away with the impression that the cargo carrying capability is more than adequate for most of my needs. I can't haul full sheets of drywall in it but that's not something I need to do very often so it doesn't bother me. Bikes, wheels, camping gear, CostCo trips, etc.....the X1 should be more than adequate.