Here's a review of a game that offers the first few levels free, with a single IAP to unlock the rest of the game. Everything is clear and above board and seems to follow the Doom (etc) model - playable game and pay to unlock the rest.
Sure, if the game isn't properly up-front about being a limited time demo, it rightfully feels tacky. But if you "bought" it by clicking a "try for a day" button, why would it feel tacky?
http://www.pocketgamer.co.uk/r/iPhone/Glyph+Quest/review.asp...
Note that this reviewer calls it "tacky".