Is it worth getting an agent if you're self-publishing?

I think there are some arguments for getting an agent if you plan to self-publish, even though it can mean giving up 15% of your earnings.  The biggest reason I think it's a good idea is:

• an agent who knows the market can give feedback on your book and suggest changes that mean it sells more, and makes more money.  In the end this could mean more profit, even after the agent takes their cut.  The biggest trap for newbie authors is writing a book that doesn't sell well because they haven't researched the market properly or have rushed into print without doing enough editing.  100% of not much is still not much!  Whereas 85% of a quite a bit is a lot more in the hand!

That said, it needs to be a good agent who's willing to read the manuscript and knows the market.  What do other writers think?