i have an 8 on the commuter and an 11 on the mtb - i've not had any issues with the 8 and only a slight oil leek on the 11 which was easily fixed (although it ment spending £15 on the correct tool).
both were a little clunky on teh fist 1-200 miles, but soon smoothed out.
as for the op's questions...
8 or 11 speed (Scotland, lakes, Rivinton, Peaks riding)
the 11 is a little lighter than the 8. the gear ratio is a bit bigger on the 11 and also more even. the cable release is much much better and macks rear wheel removal much easier. also the shifter is the right was around. its also smoother - thanks to conical gears inside.
Are they lighter or heavier than "normal" gears, cassette etc.
both the 8 and 11 are comparitive to a 'standard' tripple chainset and mechs setup, so less gears for the same weight - the 11 is a little lighter, but not noticably so. The placement of teh weight is at the rear of teh bike which is noticable when you fisrt ride one, however you soon get used to it and its not a problem.
Can the rear wheel be removed relatively quickly? (for transport in the boot)
obviously this is slower then a standard qr setup i'd say it takes about 1min with practice - and it depends on the frame - i find slot dropouts harder than an ebb setup. the 11 is much easier than the 8 as it has improved cable management at the rear. Also when you put the wheel back in you have to make sure that the shifter is in 4th (8speed)or 6th (11speed) gear as this is the 1:1 drive for the hub. also a 14mm spaner bebomes part of your esential trail tool kit.
Alfine, worth it?
ulitimatly, i guess if your thinking about buying into hub gears, you are looking into the plus sides as well, and your op is about the negatives your thinking of. For me they aredef worth it. i hardly have to thing about my gears, just lube the chain every now and then. For me the + out weight the -.