I normally call myself a blue Liberal (although my support for the party has been much less enthusiastic since Martin stepped down as leader). I've tended to dislike the NDP for the same reason I dislike the Harper-led Conservatives: they're extremists who govern based on ideology, not based on facts and rationality. I prefer a moderate government that enacts legislation that makes sense and can be supported by research and data. In previous elections, I would have ranked the parties thusly: Liberal > Conservative > NDP. Harper's government has been so thoroughly distasteful, though, with their utter contempt for any and all research, science, statistics, and information-based decision-making, that I would strategically vote for the local candidate in my riding with the best chance of defeating the Conservatives, even if that meant holding my nose and voting for the NDP.
Thankfully, I live in Calgary Centre (Kent Hehr's riding), so my decision this election is an easy one.