I've had a pretty extensive internal debate with myself about this. One day I want to get a functional shark fin, one day I want to keep my antenna in the glass, and another day I want to get one of those antennas that come stock on the 07's. Before I upgraded my speakers I used to think that it was my ANTENNA that was causing my radio reception to be so horrible.
When I had this "head unit that didn't actually fry on me debacle," I was using the HU from an F150 for about 3 days. The reception with an F150 HU was CRAP in comparison to the Fusion's unit. CD playing quality was the difference between night and day. The quality of the stock Fusion HU was a heck of a lot better than I thought it was.
Now that I upgraded my speakers and added a pair of 6x9's to the rear, I realized that my SPEAKERS were actually my shortcoming of my reception "sheetiness" and NOT the antenna. My stock HU with the upgraded speakers can go toe to toe with any aftermarket HU when it comes to playing regular radio feed and CD feeds. I just don't see the quality of the aftermarket units being any better that you can actually tell an AUDITORY difference. It is my OPINION that, unless you put some SERIOUS high end speakers in your car, anyone who says they can tell a difference with basic or even mid-level Boston Acoustics, Alpine, Polk, or Kicker speakers is probably trying to justify spending $100 or more on an aftermarket HU when it wasn't completely necessary.....of course, unless you wanted a DVD/Nav or HD radio. No offense to anyone, but that's my 2 cents.
So, now I only want to get a shark-fin antenna for "the way it looks." But, after seeing them on some of the Euro cars, I am just fine with my current antenna that's in the glass and don't think I'll be changing it anytime soon.