And why shouldn't it be?
As recounted at the Trek Movie blog, the Alamo Drafthouse was filled with Star Trek fans who'd come believing they were going to see a print of Star Trek II: The Wrath of Khan on the big screen, to be followed by 10 minutes of footage from J.J. Abrams' new reboot. New Star Trek writers Roberto Orci and Alex Kurtzman and producer Damon Lindelof introduced Khan, but the film seemed to malfunction after only a few minutes. As everyone's scrambling to fix it, in walks Leonard Nimoy, holding a film canister and asking if everyone wouldn't rather just watch the entire new Star Trek film instead. The audience explodes in a simultaneous nerdgasm.
Given those circumstances, how could the response fail to be deliriously enthusiastic? Even non-Trekkers such as myself hear it and think: Wow, that sounds really cool. And while I don't doubt that the new Star Trek probably is good, I think we shouldn't put too much stock in these early reports from a screening that was designed to elicit positive reaction. The real surprise here would be if they'd done all that -- the bait-and-switch, the surprise Nimoy appearance, the unspooling of the film a month early -- and the audience had said, "Meh." That would have been noteworthy. But giddy enthusiasm after an event whose sole purpose was to produce giddy enthusiasm? Not exactly earth-shattering.
Don't forget, the early word from secret screenings of highly anticipated new movies is ALWAYS good. It was the same way with Watchmen, WALL-E, and Transformers. Sometimes those early reports are backed up by calmer audiences later on; sometimes the fervor cools and everyone realizes, wait, those giant robots just spent 20 minutes doing slapstick in a flower garden (for example).
Like I said, Star Trek may well be a fantastic movie. I'm just saying to take the early reports with a grain of salt. The reaction of someone who saw it as a surprise, complete with Leonard Nimoy appearance, will probably be different from that of a regular audience member who buys a ticket and knows what he's getting into beforehand. Unless Nimoy's going to turn up at all the regular showings, which would be awesome.