I usually like to get the .net and sometimes the .org of my product, but seldom go beyond that. While it's in the domain registrar's interest to sell you .info, .biz, .us, .co, .whatever else, I don't see a lot of benefit. The one exception would be if you are marketing to a non-US country (eg. if marketing to the UK, perhaps get bot the .com and the .co.uk).
If a domain doesn't pass the "ear test", where someone hears your name and must be able to find your domain on the first attempt, I'd definitely get other popular variations.
If you do buy a lot of variations in the beginning, be sure and forward them to your main site while tagging them with Google Analytics. That way, you can check GA and see which ones send traffic before deciding if each is worth renewing. Shameless plug but I wrote a blog post on how to do this at http://www.keepingitrural.com/leverage-unused-domain-names-b...
Forget about the misspellings, you're not going to lose a lot of traffic to them, people will soon realise they're in the wrong place. Save your money for better things like advertising and servers.