Since its release, they've done some work with the sprites and graphics as well as some performance fixes. They also removed some dev items that were accidentally left in the game. ("Zapinator gun" whoops!). Its sold over 50,000 copies on its release day and there was a peak of 17,121 players playing all at once on the Steam's Game and player statistics page.
I myself have started playing the game and I gotta say, its pretty interesting! Its very much worth the $9.99/£5.99 that I paid for it.
Unfortunately, it is only available for the PC. It would take a long while to code the entire game to support Mac and other systems and the developers state that they plan on supporting these systems in the future.
Mac users may however use Bootcamp or Parallels to run the game. (That is, if they have a copy of Windows XP/Vista/7 etc on hand.) I haven't had much luck using Wine to run the game since it cannot install .NET framework 4.0 for some odd reason.
Other than these faults its very much worth buying. What do you guys think about terraria so far?