I answered this before but maybe you didn't see it.
What is the purpose of the web site and who is the target audience? If you are trying to sell something, or just deal with the general public .COM is the only choice. How many times have you mistyped someone's email address because it ended in .net or .org? You mistype the email address because you have been trained, since the day you first went on the internet, that all internet addresses end in .COM.
A .COM address is reliable, trustworth, and normal.
The .NET and .ORG addresses are odd, but acceptable. Nowhere near as good as .COM though.
The .EDU and .GOV addresses don't count since you can't really get one of those.
Any other address is suspect. It is not normal. Why trust something strange? Other addresses scream "I couldn't do better than this. I'm desparate, so this is what I got."
The entire web has been training your potential customers that all internet addresses end in .COM since the web began. Can you undo that training? Even if you could, is it worth the effort?
In addition to the answers posted on @jimg's question, I would add that it's really a question about marketing and branding. In the answers to jimg's question you can see how deeply-rooted .com is associated with a business considered worthy of entering your credit card info into. In other words, ".com" is itself a known brand that all .com domain names inherit.
You could go with another extension of course, as there are businesses that do use other extensions successfully. But it is an uphill battle from a marketing/brand trust point of view. In other words, you start with negative points and work your way back up to zero, where all the other .coms start.
I think it is theoretically possible to attain success with a non .com domain, but why give yourself a handicap at the outset?
Historically, the .com suffix means Commercial, and therefore the connotation to a .com website is that it is a site from a company, where something is being sold.
That has changed a bit in the past years. Most blogs, socialmedia sites, communities are .com
However, there is an agreement that if you are creating an online community you should call it .net; If you're creating a NPO, you should call it .org; If you're creating a radio website you should use .fm, or .tv for online tv channels.
Older versions of IE (I believe version 6) will assume .com if a user forgets to include the top level domain when entering a URL in the location bar.
Some companies have gotten around the .com problem with creative domain and company names: bit.ly, del.ici.ous, etc.
I'd say that having .com is the best choice as it is the norm.
Most legit businesses have .com.
Personally, when I see .info, .cc, or .biz, the first thing that pops up to mind are scams and sleazy affiliate marketing.
Even if you want to use .info or .biz for your branding you should buy the .com name as well simply to defend yourself from cybersquatters taking it. Then you either pay them a lot to get it or your prospects get confused when they end up on the cybersquatter's .com page which is full of junk.
The advantages of .COM is that if people only remember the name of your "product" and not the url, it is way more likely that they will go .COM then .NET
Also, some users like to use Ctrl + Enter when they type in the address bar, which automatically inserts a .COM to it.