I saw it today...
My opinion? Alien Encounter was better, but not by a whole lot. I don't think nearly as many kids will freak out. The Stitch animatronic was neat, but not as great as Disney would lead you to believe. However, I was very impressed with the cannons. Those things can move! I was also very impressed with the sergeant. I think he's even more impressive than SIR. He does look a bit goofy, but it fits his character. The main show is interesting and fun, though there were a couple dead spots that for now I'll attribute to the ongoing tweaking of the show. The end was a bit lacking.
All in all, I think they've got a hit on their hands. A good replacement for Alien Encounter, but not the best it could have been.