I've had my Apple Wireless Aluminium keyboard for about 18 months. It was love at first sight when I got it out of the box because it's small and sleek. I type very fast, and the keys on this are fabulous; not too sensitive, but they are thin and much easier to type with, although for some people used to chunky keys it might take a day or so to acclimatise. Well worth the effort, though. Much less taxing on the hands.
Although I really like the iPad's own virtual keyboard, I do type a lot so I like to use this keyboard with that, too. Mostly I use it with my iMac, though, and switching between that and the iPad is very quick and easy. Incidentally, you CAN pair this keyboard to your iPhone, but only if you've got a 3GS or later).
I also have the wired version of this keyboard with the number pad. After using the wireless keyboard, I find the number pad feels 'odd' and in the way. Of course, if you're an accountant you might disagree. :-)
Bear in mind though, that this wireless version has no USB ports. The wired version has two. That might make your decision easier. However, having said that, some things like scanners, digital cameras etc. will NOT work if you plug them into an Apple keyboard's USB ports anyway. Me, I prefer to be wire free - after all, it IS a wireless keyboard - and use a hub for my kit.
For those who have either small desks, or a keyboard shelf and have struggled to get a keyboard and mouse side by side, this keyboard is the answer. It sits nicely in both those scenarios with plenty of room to spare. And of course, if you have the Magic Mouse too, you won't need to try to squeeze a mouse pad on because you don't need one.
Unlike previous Apple keyboards that tended to get dust and crumbs inside that fall to the bottom (which then show), that just doesn't happen with this keyboard. So if you're fussy about those things (like me) that's another plus.
Battery life was a big issue with this keyboard at the beginning. Apple soon released a firmware update to deal with it and now it's a different story. Batteries last ages.
I know it's a very positive review, but really, in 18 months I've had absolutely no issues with this keyboard. It's the best keyboard I've ever had and I've been using Macs for over 20 years (as well as the odd PC). I really wouldn't be without it now. Typing on anything else just seems so 'yesterday'.