Aye, I'm in the Beta. Gamestop has a deal they're peddling now that if you preorder a copy, you get a Beta key (limited to how many beta keys they have left). If you know for certain that you're going to buy it, pick it up now and grab one.
The gameplay tends to be much more fast and furious, with emphasis on rushes. Note that it's possible to turtle up and advance your tech as far as your economy can go, but it's difficult when your opponents don't want to do the same. Still, even if that's not your style, the game is fun, in my opinion.
Changes to make note of:
+ Vespene geysers DO run out. That is, once depleted, there's no more mining it for half resources. You have to seek out another geyser to mine if you want more. Still, in most games that I've played, about a quarter of them you don't really have to worry about it. You've either beaten the opponent or he's beaten you at that point.
+ Terrans can upgrade their buildings to a degree-- that is, they have an upgrade that increases armor of buildings by two, they can upgrade bunkers to hold two more marines and they can upgrade supply depots to have +8 supplies (on an individual basis, subject to the energy supply of comsat stations).
+ Zerg have nifty fun units that can move while burrowed (Infestor and roach). Overlords do not automatically detect-- you have to upgrade them in order for them to see cloaked units.
+ ALL the games you play will be recorded. You can go back and watch the replays at any time (unless Battle.net is down) and examine units, their special abilities and different build orders. It's a great tool.
+ You can also review the first 33 seconds of everyone's build order, compared with your own. Another great learning tool.
Hmm. Everything else you've probably read on the website. If you get into the beta, let me know. We can set up a match and I can run you through the terran and zerg tech trees, plus different unit abilities.