I've got one and it does take a fair bit of practice to get profficient with it. I wouldn't recommend it as a starter macro lens....better to work at 1:1 with a 90-100-105mm lens, then add some extension tubes to go towwards 2:1 before graduating onto the MP-E.
It's also a lens that needs flash in most scenarios, so better to factor in the cost of a twin-light, ringflash or bracket system.
Once you get the hang of it then it really is the ultimate macro rig but I think going in unprepared could lead to a lot of frustration.
Bob