Even the trade group representing professional tax preparers says off-the-shelf software is probably fine for most people working on their income tax returns.
The IRS requires that all tax preparation software meet specific standards before it is approved for electronic filing. Since tax laws change from year to year, one needs to make sure to have the appropriate package for the current tax year.
Simple returns that don't require a lot of extra forms are seen as the best candidates for do-it-yourself filers. On the other hand, small business owners, or individuals with several sources of income, or other issues that complicate the return might benefit more from using a tax professional.
In more complicated cases, someone handling their own return might miss an opportunity to increase their refund or save on their taxes.