S60 is the operating system used by Nokia smartphones, and now made by Nokia (used to be called Symbian)...it's used in various devices, including all of the N-series and E-series phones, as well as several of the "regular" phones such as the 5700.
The N75 is worth it if you can get it subsidized from AT&T. If not, it might not be, as it'll be well over $300. The Nokia 6290, however, is also a nice phone (also runs S60) and is available on Ebay for around $300 at times...keep an eye out. It's a Euro-market phone, but has both US frequencies on it, even though they don't sell it here.