Yes, they are good. I have 2 on bikes at the moment. But they are heavy, and they do concentrate the weight at the back of the bike. I don't find it an issue while riding, but when handling the bike it can sometimes be a pain. They are also less efficient than a nice clean derailleur set-up and especially single speed set-ups. I can notice this, especially in really low gear crawling as I do quite often on the fatbike. But generally it is not an issue, and if you ride in the kind of conditions where your nice clean derailleur set-up quickly becomes a mud and dirt clogged draggy mess, then all of a sudden the hub gear proves it's worth. I guess it depends on the type of riding you like to do. I wouldn't want one on a cross country race bike, but I'm happy with them on the fatbike and the tracks and trails bike.
You will probably need the fitting kit as well as the hub and shifter, but it depends what the package is you buy. Sometimes you get the fitting kit thrown in. I would recomend SJS for the hub as you can usually get them for £150 and the service is good. Wiggle are often better for shifters and fitting kits though!
I would stick with the 8. The 11 is apparently not so reliable. It does offer 3 extra gears at the faster end, but if you pick your sprockets carefully missing them shouldn't be too much of an issue. Unless you love pedaling down step slopes in top gear with a 3x set-up. As I said, I wouldn't want one on a race bike, but if you are content to cruise down hills that have you doing 20+ anyway you won't mind.
You will be able to use a normal crankest, but probably not with a standard alfine sprocket. Alfine has a strangle chainline (55mm from memory) but if you use sram or sturmey sprockets dished out then you can usually sort out a decent chainline.
For info on the chainline issue, and gear ratio choice Sheldons site is magic.
