Thats how the watermelon one started out, I was going to keg it but saw a watermelon on the counter, so I chopped it up and took the potato masher to it, ran the juice through a mesh bag and racked the beer on top of it! later my wife asked where the watermelon she bought for the weekend went.
I like a fruit forward beer, I tend to go heavy so the flavor comes through loud and clear. this usually equates to about 1 pound of fruit per gallon.
The reason I freeze the fruit is that it breaks down the fruit a bit and makes them a little mushy, I feel that this helps get the flavor into the beer, it is also easier to rack off the fruit than a puree in my opinion.