I have used Credit Karma Tax since it came out (I think this year was 3 years) and it has been a great product and is free for both federal and state filings. Doesn't support some more complex scenarios, notably partial year and multi-state returns (you can still file your Federal return with it, though).
Edited to add additional details as to what isn't supported.
I have not done taxes on both for the same year and compared the two. I have used TurboTax, as well as a variety of other online tax filing solutions, in the past. TurboTax might be better polished and provides better explanations, Credit Karma tax gets better every year, and I find the downsides to be a good trade off for not supporting Intuit.
I did my taxes in both last year (started in TurboTax and learned about their abuses - a coworker suggested Credit karma) and found the numbers to be identical. My taxes are relatively complex, more than most people. I'm very happy with the service for the second year in a row.
I admit that they don't have quite the community links that TurboTax does, and there are some situations they won't handle (for instance, income from 2+ states)
Right, I plan to move to another state in the next year or three, so I am hoping they add that ability next year- they have added new features every year, and it seems like multi-state filing would be the next biggest need on the list. You can still file your federal taxes with them, but filing state taxes separately is a pain!
Edited to add additional details as to what isn't supported.