I haven't done it myself, however a friend at a rather large forum has several advertisers using this option on Google Adwords. Most of them have been there for quite a while so Im guessing that the ROI is worth it if you target the site and audience well.

With regards the CPM level - it highly depends on the keywords. I once had a loan site and the average click cost well over $8 - so I prefer to start low with niche keywords (use wordtracker.com) where the CPM level is much lower and work your way up to the higher levels.