I've been playing this on my ipod touch since it came out. It's a lot of fun, but not very deep.
By the looks of the trailer, they are adding more stages for the WiiWare version, so that's good. Here's hoping for more classes as well.
As far as tower defense games go, this one is more like PixelJunk Monsters than Desktop Tower Defense or Field Runners. Basically, it has a predetermined path for the baddies and you just set up defenses on the edges of the path, as opposed to using your towers to make the path as well.
Some would argue that it means less depth, while others will argue that it lowers the need for pointless micromanaging.
I personally love both styles, though I'm not sure if I will pick this up again as I've already paid for it on my ipod.