I built a drop bar road frame with the Alfine hub and the Jtek bar end shifter which I really like. So far as I know it and the Versa brifter are the only aftermarket shifters yet made for the Nexus 8 and Alfine hubs. Still better off as far as shifter types go than the SRAM, NuVinci and Rohloff hubs which ONLY offer twist shifters.
The SRAM iM9 has a noticeably wider overall range than the Nexus8 and Alfine hubs, 340% versus 305% IIRC. As mentioned by another poster the SRAM gear steps are considerably more even. If you do not mind the weight the NuVinci has no steps, being continuously variable from low to high.
I have all of them, except the Nexus8, on various bikes.
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Gear Hubs Owned: Rohloff disc brake, SRAM iM9 disc brake, SRAM P5 freewheel, Sachs Torpedo 3 speed freewheel, NuVinci CVT, Shimano Alfine SG S-501, Sturmey Archer S5-2 Alloy. Other: 83 Colnago Super Record, Univega Via De Oro
Visit and join the Yahoo Geared Hub Bikes group for support and links.
http://groups.yahoo.com/group/Geared_hub_bikes/