I've used the same CPA for every filing year from 1992 forward. Prior to that, I did all my own except for one year (when I was in my early twenties). We used H&R Block that time, I found errors in their work, and that was the end of that. The last year I did my own (1991) was fairly complex, since my soon-to-be-ex and I had a small business with a large income and two different states of residence, plus a few other quirks. For that one I used Turbo Tax, but had to explain to my lady's accountant (we were filing as Married Filing Separately that year) why I'd done what I'd done. My spouse couldn't follow my reasoning, but the accountant could.
In 1992, after a 3 year search for a good one, I was fortunate enough to make contact with a South Dakota CPA who's handled the paperwork ever since. And this year (2012 income), was I ever glad...because I'd have never in a million years gotten it right. Our $$ situation is not THAT complex, but we get some royalties from mineral rights in North Dakota, and the number of forms for this and forms for that...daunting to the max. The filing copy for our records drawer was something like 3/8" thick.