I've played every edition of the game (well, actually, I didn't play much of 2nd edition), starting way back in the 70's.
My current group has been playing together for 10+ years, so we played a lot of 3.0 & 3.5 before 4e.
We were playtesters for 4e, and right away we knew we loved the system -- it fit our style of play, etc. We haven't looked back. We have always also loved -- deeply loved -- Paizo's work. Their stories, their art, their production values are dynamite. But by the time 4e came out we were so tired of the problems with 3.5 we never even considered trying pathfinder.
But as much as I enjoyed playing 4e, I love it most as a DM. I run about 50% my own material, 50% off-the-shelf. There's been a lot of talk about what's great about 4e for DMs -- monster stat blocks & reskinning, not needing a stack of books and tables at hand, etc etc etc.
I like the DDI tools quite a bit, and I know they're getting better all the time. I love what some 3rd party folks have done with them -- both for mobile devices and on the computer. (i4e app, Maptools & Power2ool.com especially). Most of that wouldn't be possible without the effort that goes into making the content available through an online database with a sensible API for 3rd party use. And that, I love that Wizards provides.
Warts and all, I love skill challenges, and have in all variations. I love that I can spend more time fiddling with a complex challenge than I do preparing for an encounter if I feel like it, and I also love that I can easily ad lib a challenge just by knowing the Easy/Medium/Hard DCs at a given level.
Right now, our group is playing through one of the Paizo adventure paths (Second Darkness), converting it to 4e, and setting it in Eberron. It has been interesting to go back and look over the 3.5 encounters (this adventure was published pre-pathfinder, I think). As much as I love the fluff, the art, the story, everything around the mechanics (because Paizo has always rocked that stuff), I'm so glad I'm converting it to 4e. And I'm incredibly happy that it's as easy to do as it is.
So, yeah. Big love from this gnome.
-j