I have several IGH bikes and have been riding IGHs for ~30 years.
- Shimano Alfine: smoothest shifting but steps are irregular. I find it annoying on a commuter, no issue in the dirt. Newest version has new guts, 501. Had it working down to -20*F.
- Nexus is good, need to be aware of the model. I'd only consider the 8r25 or 8R35/6. The 8R35 has the Alfine 500 guts, the 8R36 has the newer Alfine guts, the 501 version. Had it working down to -20*F
- SRAM i-Motion 9: it's not as nice shifting as the Alfine, the steps are regular, works well on a commuter, not up to mountain biking. Not so good below freezing, I've switched it to oil bath, should be better next winter
- Rohloff, cost is ~$1600, what's the point?
Here's a couple of good sites for IGHs:
http://hubstripping.wordpress.com/
http://groups.yahoo.com/group/Geared_hub_bikes