The top 3 spots are more prone to click fraud than the lower spots. You're also more likely to be a victim of click fraud if you bid on "online poker" as opposed to "internet poker websites taking neteller", if you catch my drift.
The trick with PPC, just like with SEO, is to have a list of several thousand keywords getting 1 visitor per day, instead of having a list of 10 keywords that get 1000's of searches a day.
Randy is 100% correct. Stay out of the top spot for sure. I have used them with some success, but I learned the hard way to not be top listed on any big term.