I did an extract Mango IPA last summer. I put 6lbs of mango in the primary after it had fermented out. The first bottle was AMAZING! the best thing I've ever tasted. However, very shortly thereafter the mango flavor turned to pure tartness, almost like biting into a lemon. I gave it a few more months in the bottles and some of the harsh tartness subsided and it became a very pleasent, albeit weird and still sour, beer. It was a huge hit with my beer club.
One suggestion. You can't overdo it on the fruit. I'm serious. I usually do 50% more than what a recipe calls for. I recently did a watermelon wheat with 1.8lbs per gallon and it was still not overpowering. If you want to taste the fruit in the finished product, use at least 1.2lbs per gallon.