Free air FTW!
For normal music, it really rocks, even at decent levels. It will not however, rock the neighborhood. If yo want good, clean, MUSICAL bass, free air is great. If you like rap, or whatever, and need thump that people can hear down the block, you need a box.
If you want to do permanant damage to your hearing, be partially deaf before you turn 30, get a box.
Oh, nice little benefit of free-air - you get to have a useable trunk. Also, does not require gobs of power, and is much harder to steal.
And no, you don't have to get all anal about sealing the trunk, just mount them to a baffle board, screw it down really good to the seat back, (maybe with some foam to isolate the two - and prevent rattles), and have at it.
If you are building a home theater, and have a spare room (attic or basement) NOTHING beats an ib setup. I really wish I had an attic or basement near my HT!