That's exactly my experience as well. (BTW, you should update your sig now that you have the 5870!)
Regarding if it was worth it to upgrade, I think the value will largely depend on Apple giving us improved graphics drivers with future OS updates. Cinebench and XBench results certainly do not show much, if any, improvement over the 4870, however, I am getting dramatically better performance in Left4Dead 2 and Team Fortress 2 (yes, I do some gaming on my MP!). This tells me that there is a real advantage to the 5870 over the 4870 that OS X can make use of, and perhaps Cinebench and XBench aren't providing real-world-relevant results.
Also, the 5870 has twice the VRAM as the 4870, so apps that make use of that (CAD, rendering, and probably to a lesser extent Aperture and Lightroom) should also see some improvement over the 4870.
I do think it was worth the upgrade, and it's good to know that I'm not the only one that noticed a slight bump in idle fan volume. A funny thing about it is that my workstation is in the living room, and the kitchen is just around the corner. We have a pretty old refrigerator, and when it kicks on, I can't hear any fans unless they're really cranked up, like around 2000 RPM.
The only game I play with this machine is Counter Strike: Source, and it seems the same, but I also raised some settings that I didn't notice before. With the 4870, I had all of the obvious things set to the maximum quality, but this time I found some more settings that I don't understand. Things that have options like something-something 4x, 8x, 16x... I bumped those all up as well, and now I noticed that the on-screen text is super sharp and fine. I only went into the settings because it looked a lot different than prior to installing the 5870, and I assumed the new card might have thrown them off. After fiddling around with them, the game looks better than ever.
I set on-screen display of FPS, and no matter what is going on, it's always either 59 or 60 FPS, and the fans don't speed up like they did before. I also set the FPS display to the "smooth" setting (2 instead of 1), which I hadn't done before. I really don't know what all those different settings mean, to be honest... just the obvious ones.
My main program usage goes to Adobe Premiere, After Effects, Soundbooth and Audition for Mac (beta), Photoshop, Illustrator, and Encore for the disc outputs. I have to say it seems to work faster with less effort with the 5870, but I'm often wondering if an nVidia card with CUDA would have been a better choice. My understanding of what I've read about CS5 and the Mercury Playback Engine's use of GPU acceleration is that it only makes a difference in Premiere. I've not had any issues playing two layers of HD footage (or one layer with effects on it) at real-time, and it's not a big deal to render out segments when it doesn't play back smooth, so it's even harder to justify buying a $1200 card like the nVidia Quadro 4000. I guess I could have bought the GTX285 when they were around, but I didn't, and I really don't know that I've missed anything.