They are mostly utter garbage, more in the Zynga 'social' angle than anything else. The majority of what's left have incredibly simple gameplay mechanics, because the control schemes open to you just don't allow any more complexity. apart from the gyroscope, which I'm not convinced anyone will ever use well, all your input methods involve you obscuring the screen with your fingers!? And can never provide feedback to the user!? Madness.
Mobile gaming is huge, and it makes money, but mobile gaming is a bit like 'social' gaming: it is not actually 'gaming', as gaming is on other platforms. These things exist side by side, one does not replace the other.