i use shapeshifter and thats what i think most people who adjust their themes use. i havent noticed any slowing down due to a different theme, my mbp still runs fast.
I can't put into words how much I adore MyTenFive. It's Leopard's unified, smoothed windows/buttons, with classic aqua elements, and without the crappy menu bar and dock. It's how an upgrade to aqua -should- look.
Are there any problems with incompatibility with either MyTenFive or ShapeShifter? I've always been afraid to try this type of program simply because I didn't want to cause a crash. Perhaps this is just a holdover from my experience with Inits in OS 9.