I have been evaluating different PGP applications trying to pick the best PGP desktop software. I use Gmail on both Windows and OS X, so I want something cross platform and free.
I’ve tried both the open source Gpg4win package for GnuPG and the commercial PGP Desktop. In my experience, the open-source applications I tried were too buggy, incomplete, and unfriendly to be worth it, especially to the non-technical user. By contrast, if you are willing to pay $99, PGP Desktop is much easier. For occasional use, the freeware mode (tutorial) of PGP Desktop works just fine. I did find a GnuPGP tutorial for OS X, but my experience with the Windows front-ends has discouraged me from trying it.
I also tried FireGPG, a Firefox extension that integrates with Gmail. FireGPG still requires GnuPG (and must be reinstalled if you don’t install that first!) but it seems to be the simplest cross-platform PGP + Gmail solution. FireGPG works well enough, although the whole process may still be too difficult for the average user, and the buginess of the GnuPG suite let me to stick with PGP Desktop.
Until something radically easier comes along, I’m going to continue recommending the free or paid version of PGP Desktop for the average user.