Star Trek will disappoint no one.
As the lights dimmed and the familiar Star Trek Federation logo slid on screen, the emotion of all those hours of watching Next Generation reruns as a kid came sloshing back into my brain, dripping out of my eyes as tears of pure happiness. I expect that it was essentially the same emotion Star Wars fans felt during the opening credits of Episode 1, but without the massive letdown afterwards. (Ha ha, suckers.)
But yes, to answer your question, there's Kirk, there's Spock, and there's everyone you expect (even Pike!). Not all of the same mannerisms are there, but if you wanted to see the old actors you'd go watch the first six movies again. This implies that Kirk doesn't do a two-hour Shatner impression, which is, of course, good news. Instead, he plays Kirk as a intelligent, brash, but friendly youngster that has confidence oozing out of every torpedo tube. And the new Spock is more Sylar than Spock, to be honest; though the rest of the casting is essentially spot on.
So long as you go into the movie expecting a "Star Trek" movie, in that there's space and aliens and action and shooting and torpedoes and pew pew pew, you'll come out happy. The movie is targeted enough toward the mainstream in that someone with zero Trek experience would enjoy it. Director J.J. Abrams also gives enough shout outs to old time staples that trekkers will be giddy at the slight nods and fanservice that say, in essence, "thank you for supporting us all these years, now here's something you asked for."
Think of it like Casino Royale was to the James Bond franchise: fewer gadgets, more action and an incredibly pugilistic lead. And lens flares. Lots, and lots, and lots of lens flares.
Kirk's Enterprise has never looked better. These guys took the original ship, combined it with some designs of the Enterprise-B, then mashed it up with Picard's Enterprise-E and then added a dash of '60s non-Trek Sci Fi. The set design, however, is almost all touchscreen (like TNG), but with a tremendously updated UI. I'd hate to call it Apple-y, but there's lots of glass and slick white finishes. Retro this is not—you'll barely be able to equate the bridge to the original's, other than the fact that the players are all sitting in the right places. Why Bones canoodles in the bridge so much instead of where he's supposed to be is still beyond me.
And the plot? The plot makes as much sense as any other Star Trek movie. There's even a very good explanation of why this movie is the way it is, which is the most I can say about that.
This is what Star Trek needs right now. After writing on Next Generation, Ron Moore went on (about a decade later) to reimagine Battlestar Galactica, a relatively realistic show (topic-wise) that just happened to be set in space. Sci Fi fans have moved on from the utopian, and what many accused as sterile, confines of TNG to a grittier, less kempt future.
That's not to say Star Trek is now gritty—it's just more...modern. And more sexy. Like when you upgrade from a six piece KFC meal to a 12 piece bucket: you're going to get more breast and thigh.
It also doesn't have any crap about the Prime Directive or any undertones about race that TOS and TNG dealt with, but it is a very good "restart" of the franchise. With this film as the base, I cannot wait to see where the franchise goes from here.
Bonus: there's a four-issue Star Trek: Countdown comic series that prequels the movie. Though, you might want to wait until after you watch to read, since it gives away a few plot points. To tell you more would be to spoil too much. It's too much even to tell you what KIND of fans would like the comic. You can download the first one here for your iPhone.