If you frequent Myspace, which is a bad habit I can't seem to kick, then Firefox is handy to have around. The site is mostly geared towards PC users, so some things will send Safari into a Sgt. Shultz-esque "I know nothing! Nooothing!" mode. Firefox tends to handle the tsunami of html garbage a little better in that instance.
I would disagree with this. I feel that Safari handles the poor code on MySpace a lot better than FireFox. FireFox regularly crashes on MySpace, I have never had Safari crash on MySpace. Most of the stuff on MySpace is flash based, so Safari can handle that easily. Also Flip4Mac only work with Safari so is one of the only ways of watching WMV in a webpage.
I am not a big fan of FireFox on OS X, it is very slow and resource hungry and looks very ugly against the OS X Cocoa interface. It has lots of extensions that some people are unable to live without, and it can display some pages Safari can't and Gecko is still one of the best.
Personally, as a pretty basic user who just casually surfs the web, then Safari is great. For power users, Safari may not fit the bill.
I love the intergration with OS X of Safari and its speed. It would be nice if there was an option for the popup blocker to block specific addresses or give an option like Firefox rather than blocking everything. However, I don't use the popup blocker anymore and I rarely get popups anyway, so no biggie.
As others have said it is just a matter of personal preference.