The "standard" ACM conference style file is one of the most god-awfully ugly designs I have ever seen. Those who know me know that I like to obsess over nice fonts, and the clunky square font stylings that I am often forced to use just depress me.
On the other hand, SIGGRAPH (the graphics SIG of ACM) has an excellent style file, which often shows up in computational geometry proceeedings. I guess it's no surprise that the graphics folks are more sensitive to the layout and presentation of a paper.
Obviously, these are subjective judgements. So judge for yourself: