TurboTax lets you go through the entire process of entering information and telling you your refund amount before charging you.
I wasn't sure if I trusted myself with the free system, so I did my semi-complicated taxes through TurboTax until I reached the payment screen, and then went and did them in the free system and confirmed that the dollar amounts matched. Then I submitted through the free system.
Next year, I'll probably just trust myself more, so Intuit doesn't get my data.
I tried to go that route and my TurboTax refund was much bigger than FTU. I wanted to transfer all my answers to FTU, but TurboTax wouldn't let me go back and look at my individual answers without starting over. Seems like they got wise.