I'll be able to give you better feedback soon as am awaiting my SRAM hub geared wheel, what I've gleaned so far, I've only looked at top of range in each:-
Rohloff - wins every comparison test hands down, widest range of gears, changes under load, reliability is amazing, easy wheel removal with some models. Negatives, breaking in period, gears run smoother after about 1k km, have read some that claim a noticeable drag effect after this, but many more that say that's tosh. Heavier, depending who you believe could add as much as 2lbs to bike. Gear spread can mean that you have some you never use at either end. Twist grip shifter that won't fit all bars.
Shimano Alfine? - smoothest operation of all of them according to reviews, not as good a range, or spread, of gears, 8 in total. Problems of reliability and seals from earlier models have been solved(according to Shim) but need more data and time to verify this. Pain in the butt to remove rear wheel, big negative in magazine review. Easier to source and, if need be maintain plus cheaper.
SRAM I-Motion 9 - this is what I've ordered, not so easy to get hold of and whole wheel build and fitting is going to be approx £400. Range of gears good, my chosen setup will run from 29.5 inches to 100.2 inches. Some concerns over size of gaps in upper gear ranges but will depend on type of cycling you do. Reviews I've read have been very positive re reliability, not as smooth a change as the Shim and some doubts of consistency under load. I only want it for commuting and social rides, I don't need top end race type gear. Rear wheel very easy to remove but servicing may be an issue.
The useage your putting it to is key, if you're a serious racer, MTB or road, they may not be for you, though the Rohloff is rated for MTB racing. Looking at how many people commute with SS, Fixed or a 3-7 hub gear range, then there must be something in it. I want less fuss but then am doing a lot of miles in all conditions.
I'd read up on Sheldons pages(you can spend hours on gear calculations), the Velo ones, look at these and other forums, some stuff on YACF and CTC.
I do still plan on my next bike being Rohloff specific but expect to pay in the region of £2-2.5k, couldn't quite justify that just yet....good luck.....Al