The .com, .org, .net are extensions that people are familiar with. So for example If I was searching for Sears, I would automatically type Sears.com - Has the say old habbits are hard to change. But the other extension are ok if you MUST have that domain name.
But there are other way you can get the same domain with a little modification. For Example I want the domain "big Blue Cat" because that is the name of my company. If BigBlueCat.com is not avaiable, then look for the .org or .net. If not available, then you can try a variation of say Big_Blue_Cat.com. To me this second variation is easier to read. And it still reflects your company.
In internet marketing like we are doing, I personally don't think it makes much difference. But is is best if you can to go for the .com, .org and .net extensions.
If you are using PPC for your traffic source, I don't think it is any big deal if you use other extensions from the big three.