I've been interested in an Alfine hub along with a few other people on here it seems but there doesn't seem to have been a huge amount about on how they work in practice.
A mate of mine has recently got a bike with an Alfine hub and I've had a go on it around the carpark at work so I though I'd just post my initial impressions.
First thing is that the shift is very smooth - no clunk, just a smooth change from one gear to the next. The 'reversed' shifter is annoying for me, being used to original rapid fire but I guess that I could get used to it and despite it being a bit clunky cosmetically, it feels just like any other Shimano shifter in use.
One of the key things I wanted to know is how an Alfine hub works when you mistreat it - ie shifting under full load - as this is what tends to actually happen out on the trail at awkward times.
I put the bike in a fairly high gear and then with the brakes dragging and pushing hard on the pedals, tried shifting to an easier gear. As I suspected, it wouldn't shift to the easier gear until I lifted off the pressure on the pedals a fair bit, at which point it shifted nice and smoothly. The same was true for trying to shift from an easy gear to a harder one while under pressure (though I guess that this is less of an issue unless you're sprinting!).
To compare, with a geared bike while it's not something that you'd normally want to do (shifting under heavy load), I reckon that the geared bike would have shifted under the same conditions though it would probably have been a bit clunky.
I guess, that for me, it confirms what I expected - the Alfine is perfectly useable and seems to work well but it may cause problems while out riding on the occasions where you've been caught out in too hard a gear and need to shift quickly to an easier one to avoid stalling. It hasn't put me off getting an Alfine but I guess that I'm not quite so keen to have one as I was before.