To clarify, I do think that this post gives good alternatives because most spam is not targeted. However, you must do something like this if you're a big site or a small site who pissed someone off
The reasonable thing to do would be to initially create challanges with multiple levels/difficulties so you can quickly change the mechanism when you are really targeted.
For my personal blog I managed to be spam free with a simple question/answer pair for 5 years. Took me a minute to implement and leaves my user data where it belongs.
