The ads should be a lot less intrusive than most ads, particularly uber-intrusive-and-annoying newspaper ads. TechCrunch notes:
The ads will be marked “Sponsored” and a rate limit will ensure users see no more than 1 Sponsored Story in the news feed per day. They’ll only feature stories about friends or Pages that users already like. Users won’t be able to opt out of seeing Sponsored Stories in the news feed or having their activity used in them, but they will be able to ‘x’ out individual ads. The ads won’t immediately appear in the mobile news feed, though Facebook is considering the idea as we discussed earlier this month.
Some people are already upset about this, but would you rather pay to have an ad-less Facebook or have some ads thrown in?
My guess is that the ads will be fairly targeted and most users won’t care. They may even show how Web ads can be done well.