I use the blue. I just really like that color better than the grey. Grey makes the desktop look cold and unwelcoming. The aqua just makes it look more welcoming and enjoyable.
I generally prefer Graphite, but I tend not to use it because some applications do not adhere to providing two color types for their buttons. Safari, I'm looking straight at you. Plus, as GimmeSlack12 said in that other thread, differentiating between foreground/background windows is not as obvious in Graphite.
I hope they work on/implement theme support in 10.5, but I'm not holding my breath...