Words with Friends® is a great Zynga game... I've been playing it on my iPhone and Google Nexus (with friends, of course) for quite a while and love it.
Naturally, I bought the paid app -- the advertisements on Zynga are particularly obnoxious: they show up early and often and hang around way too long before you can skip them. They essentially force you to buy the paid app to avoid the ads.
Now Zynga has reached a new low: on the paid app, they are inserting advertisements for their own paid extensions to the game. EVERY SINGLE TIME I tap "swap", I get a delay followed by a pop-up suggesting I purchase an extension to find the words that are available with my letters. Shame on you if you think that's a great way to make a game more enjoyable and playable!
I presume this is all perfectly legal (their Terms of [Dis]Service statement spans 7640 words / 47391 characters... nice easy reading, eh?) but it certainly offers plenty of opportunity for a more user-friendly game to get some traction.
Zynga: I'm still waiting for a response to my complaint; when are you going to fix this?
Update: To their credit, Zynga did remove this feature, a relatively short time (weeks?) after this blog post (but probably not because of it :-). My guess is they got flooded with complaints and decided to respond appropriately.