A few thoughts from a "switcher":
After 20 more years playing D&D, I grew frustrated with 3rd Edition, searched for a new system and found HARP: great system at the start, takes time to figure some of the things but easy to go on after that, great flexibility, wonderful magic system, fast PC and NPC creation.
And so I went back to try to figure out how to build interesting modules, instead of calculating endlessly the best combos for my NPCs and dealing with my hungry players always wanting the latest (and more powerful) Prestige Classe, that doesn't fit at all in my setting.
From there, I also decided to go back to an old setting, Middle Earth, and convert HARP rules to fit to this wonderful universe. Let's say conversion is a breeze, as Classes, Training Packages, Cultures and even Races are easy to create or customize.
Games are flowing wonderfully since we switched, with nervous and nasty fights, some nice RP opportunities mixed with some rule aspects we had forgotten a bit: finding herbs or dealing them, approaching groups to get some specific Training Packages, etc.
Thanks Rasyr, because after having written the rules, you support them by your (when do you sleep?) presence on the boards. That has been one of the facts that made me switch, and now my group is hooked and we're hoping for a great future for HARP.
(PS: excuse my poor english, native french here

. A shameless plug for french speaking readers: here's my website with the Middle Earth - HARP conversion:
http://www.harp-tdm.com/).