I'd sometimes had lingering thoughts about the buttons in QuickTime being too large, and SlimTime Pro not only affirms that, but does away with the problem by greatly 'slimming' down the interface, making the buttons a fraction of their former size. Don't get me wrong, the buttons are still very clickable, they're just now at a reasonable size.
My one and only gripe with SlimTime Pro, however, is unlike QuickTime's default appearance (and iTunes for that matter), the bottom bar is no longer draggable; I'm very used to being able to grab just about anywhere and drag the window around, but instead I'm forced to go up to the top of the window and grab the bar to move it around.
That aside, SlimTime Pro is a huge improvement over the bloated interface that resides at the bottom of every QuickTime window, and I'm more than willing to look past that tiny caveat and appreciate how much nicer and unobtrusive QuickTime becomes =)
However, it should be noted that if you have trouble getting the installer to run (I was greeted with a mysterious lack of any kind of window), I recommend installing the latest version of QuickTime (currently 7.6), and then running SlimTime Pro's installer.