I haven't seen one in play, either, but they do look weak to me, especially at low levels.
The limited number of spells known and spells per day is at least somewhat mitigated by having a lot of the mysteries be more powerful than spells of a similar level.
More than anything, I think it's the path system that bothers me about the class. Being stuck with paths means that you'll often have to take a lower-level mystery that you don't really want in order to get a high-level one that you do.
It also makes shadowcasters suffer severely from the problem that 3.0 sorcerers had: the paths that are good for beginning characters are not those that are good for high-level characthers, but there's no way to swap them out. For example, Flicker and Killing Shadows are both great apprentice mysteries that will remain effective at high levels. But, if you were to start to first level and work up their paths, the only mystery you'd have to use in combat before you got Killing Shadows would be Voice of Shadow, which isn't even very useful. Low-level characters who want to be useful in combat are basically forced to take either the Dark Terrain or Touch of Twilight paths, even if they'd prefer to have other 3rd-level mysteries.
Being able to freely choose mysteries (and just having a set bonus feat progression) would make the class a lot easier to play from 1st through 20th.