I've been running MIUI for a few days now and I'm a heavy user of everything in the UI.
I had a few problems with the camera at first, now I don't for some reason. Used to be slow showing gallery from camera app like you stated, I re flashed it twice because the new version came out with the CM7 camera but I didn't like it so I flashed back to the MIUI v3 without the CM7 camera.
I've had less problems with the entire rom in general since I uninstalled setcpu and didn't OC the phone. It's still crazy fast and now I don't have as many issues, used to freeze like 2-3 times a day and I had to reboot the phone because of it. Now that doesn't happen since I uninstalled the OC stuff.