Potential Subwoofer Mounting Spot

I search around and didn't find any threads about it, but that doesn't mean it hasn't been mentioned before.

I am buying a pair of subwoofers and an amp to add to my sound system.  I don't exactly want to sacrifice trunk space if I don't have to, so I was thinking about mounting them in the floor of the trunk.  Has anybody done this before?  Is it difficult?  Does it sound crappy or is it an okay location for subwoofers?  Like I said, sacrificing trunk space isn't out of the question, but I'd like to save that option as a last resort.