I have desktops, a laptop, and just bought an ipad2. I'm still getting used to the tablet, but I'll try to list some pros and cons.
Desktop - bigger monitor, some applications are faster, larger cache (don't need to reboot as often). Other than that, not a lot of difference. Oh, and I can run stereo and my game console through the PC and monitor for sound and screen.
Laptop - portability is the big plus. With the size of storage and RAM nowadays, as well as processing speed, you probably won't notice a lot of difference in performance from a laptop to a PC. Depending on what you buy, price differential isn't all that great any more, though laptops are still somewhat pricier than PCs.
Tablets - as I said, I looked long and hard at different tablets, but went with the ipad2 in large part because of the size. It's got a larger viewing area and it's a beautiful picture. I'm pretty pleased with it so far. I find that as an older person, I'm not as enamored with the touch screen as maybe some people are, but I also bought it because it can be accessorized with peripherals such as keyboard, mouse, and players.
One thing I didn't realize was that the ipad is itself a peripheral to your PC (or MAC). I really thought I was buying a stand-alone computer; self-contained and all that.
One thing I did do was buy a Logitech keyboard cover. It's a brushed aluminum cover that doubles as a keyboard. About 70 bucks, but to me, well worth it, both for protection, and for typing. It's a little weird because it isn't set up exactly like a standard 101 keyboard, but I'm getting used to it, and am really happy I bought it.
Which brings up an important point (to me, anyway). Effectively, I've got a netbook. I pop the ipad out of the cover and stand it up in the groove at the top of the keyboard area, and for all intents and purposes, I have a netbook, which is what I'll probably buy if my laptop ever gives out, and which I recommended Mrs BOP buy, since she's still trying to decide.
Hope that helps.