I've been running a nexus redband hub for a while now which I got really cheap off ebay. The redband is distinguished by (suprisingly) a red band around the driveside of the hub) – I think the pic below is too muddy to show it!
I don't have any trouble with it at all. It appears to be well enough sealed and the gears are very smooth. Pretty heavy compared to gears I reckon, don't know what the comparison with the Alfine is like.
Try doing a bit of digging on google. I did before I fitted it and apparently the red band version was unofficially OKd for offroad use in the USA (obviously warranty voided but hey) and alot of people were experimenting with it prior to the alfine coming out.
Take a look at this discussion on mtbr for example.
Obviously the problem you have is brakes. As mentioned in the above thread you can get disc adaptors if you hunt hard enough. Mine came from Germany thanks to my GF who happened to be working there at the time.
The shifters for the Alfine work with it although I still use the supplied twist grip and get on fine with it.
Anyway, a couple of pics of mine out in the Peak district: