I've used credit karma for the last 2 or 3 years - it's very easy if your taxes are simple (e.g. a job change mid-year, a spouse, itemized deductible, all very easy). It was even ok when my taxes included some weird forms last year because a previous employer was sold and my options were worth a few k - had to figure out where to enter things, but i paid roughly the amount of taxes i expected to so i assumed i filled out karma credit's app correctly.

Overall i think CreditKarma is fine if you don't mind that you're sharing data with a company that would love to middleman you into signing up for a new credit card or something. I find their emails pretty easy to ignore and it's felt like a fair trade thus far, though I may look into the IRS offerings this spring and see what else is available.

edit: in California.