I've been playing around with an Xbox 360 for a few weeks -- if you have an HD television, it's definitely worth checking out. The graphics and interface on a 720p TV are amazing (I haven't seen it on a 1080i, but I can only assume it's just as good if not better.) Kudos to Microsoft for the full wireless controller support -- with the couch now farther from the TV than ever in most homes, it's about time. Support for Internet play is also great -- although the initial batch of games are probably not utilizing the system to its fullest, nor do they offer the quality of design for online play seen in say, Halo 2.
However, if you're not using a large-screen HD television, it's probably not worth upgrading quite yet; my guess is that on an s-video connection, the marginal gain in quality is probably not worth the money. With the price of 42" HD televisions dropping 30% or more each year, it's time to upgrade the display if you haven't already.