There are a large number of new companies that are using .io domain names. I understand that a large part of this is due to good availability of english words, but wonder if it is a fad similar to the days of .ly domains. Do we eventually end up back at convoluted .com domain names until the next "cool" TLD?
The way I see it, if your product is technical then .io is fine. But if you eventually want to market to every tom dick and harry then you better get a .com. Another strategy is to get a .io until you can afford a .com. Also, if you can market your website to achieve a million users on .io domain, you can raise enough money to buy the .com. My 2 cents.
I think that .io domains are popular for a few reasons, they are geeky "input/output" and they somewhat resemble a 10 (binary 2). Of course you mentioned the availability aspect which is huge. I personally own two.io (not sure what I'm going to do with that) so I may be biased :)