I grabbed Orcs Must Die! on launch and am really enjoying it (i spent a good 13 hours - according to Steam) playing it off and on over the weekend - and by off and on I mean 3-4 hour stints.
I've completed the 'normal' difficulty with pretty much a 'perfect' (5 skulls) for every level. Am just trying to get the last two levels I don't have 5 skulls on completed before I move onto nightmare.
It's one of the first games of this sort that I actually go back and replay levels with different trap combinations. I think it's mainly because it's so graphic and varied that I don't feel so much like I'm just re-hashing the same old content so much - although this might change.
I'm waiting with baited breath to see what their DLC plans are like (whether or not they're going to be expensive or not pretty much).
There isn't a huge amount of content but if you like the style of game then I highly recommend it - heck, at the price point I'd recommend it even if you didn't like it. If you enjoyed the demo (not sure what's on it) i'd say you will enjoy the full-game. The game really opens up the more you unlock (weavers, which are basically skills, and additional traps/trap upgrades).