I wouldn't go so far as to say I tend to choose style over substance when I buy games, but I definitely find the former more important than the latter from time to time.
Case in point: My decision to pick up a copy of the NanaOn-Sha-developed PaRappa the Rapper.
this commercial, though, I decided to give the genre a try--due in large part to PaRappa's paper-cutout aesthetic.
Thankfully, I quickly discovered that the game was not simply another example of "style over substance." Instead, it was an example of "style marrying substance and then populating the earth with a multitude of stylishly substantial offspring." (Or something like that.)
Sure, on the surface PaRappa seems to be little more than Milton Bradley's Simon wrapped up in a pretty polygonal package. Scratch that surface a bit, though, and you quickly discover that this game offers brains as well as beauty.
See also: all of the previous '10 video games that made my life gayer' posts