I have an iPod Touch, an iPad2, an iMac, and a MacBook Pro. I have used all for gaming.
When we're playing 4Ed, the iMac usually gets used by whomever has forgotten their character, so they can access DDI.
I have found that the iPod & iPad actually do enough for me to use for gaming in general, though. I don't have any special apps: I do my PCs, NPCs and even adventure & campaign write ups in the Notes app that comes with them. Sometimes, I'll access an online resource, like a SRD.
Were I shopping for a device specifically for gaming, though, I'd probably still buy a laptop. Some of what I do would be easier or neater or more flexible on a laptop, no question. For instance, I can type a hell of a lot faster on a standard keyboard than I can on a virtual one.* And then, of course, there are all the things laptops can do that tablets can't quite.
But the iPod & iPad do enough that I really haven't needed a fully functioning computer, for reasons stated above. The iPod, partly despite its small size, partly because of it, ate away at my use of the laptop at the table. In fact, dating back to my use of a Palm Tungsten PDA, I haven't had a paper character sheet, and most of my campaign design notes were taken on that device. And honestly, I haven't used the laptop for gaming at all since I got the iPad.
* I can still type faster on a virtual keyboard than most people can, using a touch-typing technique, but without the proper physical feedback**, my fingers drift ever so slightly, leading to fun with AutoCorrect...
** why did Apple programmers waste lines of code putting virtual guide blisters on the virtual keyboard's "F" and "J" keys?