Well it all depends on how in-depth your situation is. Software like Turbo Tax and other similar products are great for the standard W-2 entrys and a few deductions. But for more complicated returns with a Schedule C for businesses, K-1 forms for partnerships, investment gains/losses, etc its always better to go to a tax professional. They know the ins and outs of each situation and will not miss things that you could easily do by completing them yourself.

If going to professionals, I found some tips from the IRS for choosing a tax return preparer. Here's a list from that site.

• Ask about service fees.  Avoid preparers who claim they can obtain larger refunds than other preparers, or those who guarantee results or base fees on a percentage of the amount of the refund.
     
• Plan Ahead. Choose a preparer you will be able to contact after the return is filed and one that will be responsive to your needs.   
• Get References. Ask questions and get references from clients who have used the tax professional before.  Were they satisfied with the service received?   
• Research. Check to see if the preparer has any questionable history with the Better Business Bureau, the state’s board of accountancy for CPAs or the state’s bar association for attorneys. Find out if the preparer belongs to a professional organization that requires its members to pursue continuing education and also holds them accountable to a code of ethics.  
• Determine if the preparer’s credentials meet your needs. Are they an Enrolled Agent, Certified Public Accountant or Tax Attorney? Only attorneys, CPAs and enrolled agents can represent taxpayers before the IRS in all matters including audits, collection actions and appeals. Other return preparers may represent taxpayers only in audits regarding a return they signed as a preparer.