Microsoft currently offers two plans for XBL: Monthly ($5.99) and yearly ($49.99). You get 12 months of XBL with the purchase of one of the starter sets (there's a Halo 2 one coming for 79.99). Certain titles
also come with a free two month trial.
Is it worth it? [censored] yeah. It's the most polished and feature rich online service I've ever seen.