Personally, I would only go with a .com. If it’s not an option, I’d go with a different domain. With that said, I have had a software firm that only has a .net (we didn’t obtain a .com for Picasa.net until right before we sold Picasa to Google) and it did fine, but a certain % of users will look for you at the .com address and never find you or will find a similar service depending on who owns the .com. It’s hard to know what that % will be for your particular offering. You can bid on Adwords for the domain with .com and try to push those folks to your non-.com page, but buying .com keywords via Adwords doesn’t always work out.
There is a real credibility issue when a business doesn’t hold the .com, and it can make a significant difference in results depending on the type of business. It doesn’t mean you can’t still be successful using a non-.com, but why add more obstacles to success than one already faces if you don’t have to? One site that can help when trying to find a .com that fits with the words you want to incorporate in your domain is NameBoy. It’s not great but it can at least help spark some ideas.
