I was a HUGE fan of Ultimate Spider-man when it first came out. In fact damn near all the Ultimate books were great until Jeph Joeb ran the Ultimate universe into the dirt with Ultimatum. I've kinda followed the new Ultimate Spider-man on and off but after Ultimatum its hard to care all that much about the Ultimate universe. This got my attention though, I'm a sucker for multiverse stories and this has shades of silver age DC Earth-1 meeting Earth-2 in it. I think I would be more excited if it was still Peter Parker in the Ultimate universe, but this is pretty interesting too.
As for the story of issue one not much to tell, it's Bendis after all so it moves very slow. But if your a fan of Bendis' Ultimate Spider-man you'll be a fan of this. All that happens is the regular universe or earth 616 as its called Spider-man encounters Mysterio working with some sort of trans-dimensional do-hicky. Unbeknownst to Peter this is actually Ultimate Mysterio. During the fight Peter gets knocked into Ultimate New York. We spend a few pages of Peter confused until he runs into Ultimate Spider-man. Both say "No way." Continued next issue.
All I can tell you is if you like Bendis' Ultimate Spider-man you will like this. If you don't you won't. And if like me you liked Ultimate Spider-man and multiverse stories you will love this.