Last year I tried out all the major offerings, and TurboTax was the only one that worked without any issues. TaxAct didn't even manage to fix their broken electronic import of broker data until some point in March or April.
I understand that I could have waited a little bit longer with filing my taxes. But the fact that key functionality is missing from a product doesn't give a lot of confidence in that they have enough resources to handle their job.
The only feature from TaxAct that I'm missing from the online version of TurboTax is attaching PDF files for transactions. With TaxAct I can report aggregated transaction data and just attach the PDF from my broker which is then automatically included with the efiled return. With TurboTax that's not possible yet and I'd need either list transactions individually (which is easy due to electronic import) or send the itemized broker statement by mail.