I wouldn't buy it for at least a few months... Think of every other major game release you can recall. Were any of them successful immediately or were there a jillion bugs that had to be corrected before the majority of people could play the game and enjoy it? When you buy the first release of anything you are a beta tester.
Look at the "OT: Do Not Buy Warning" thread just posted here to day about SimCity 4. I bet there will be a similar thread about MOO 3 next month. Summer is likely to be the earliest that a 'non-beta' MOO 3 is available.