Firstly I'm 5'11, 32 inseam and 11st. I've been paddling 2 and a half years and am pretty average (Do grade 4 not really up to 5). I brought a witch because I needed something for everything, I wasn't disapointed.
It river runs well (I have a friend who tackles grade 5 in his regularly, but he is very good), it's fast enough to get through most problems, even if you are on the heavy side for it, It carves nicely across eddie lines and is easy to put where you want it. The only problems i found were that carrying a used throw line in the back changed how it handled noticably for the worse and that if you aren't on top of it, it gets away from you easily.
Surfing is a dream, it is just so fast. Playing, on waves it's great, everything is easy it carves agressively but is easy to control releases on demand and spins till your dizzy. In a hole it is good but there are better boats.
On the flat. Squirting it does easily enough and the front end is easy to get down, once you've got used to the edges, the back end isn't so easy to keep control of as other boats but it isn't that bad.
Its not the best boat at any one thing, but it is the best if you what one boat for everything. It's a simple boat, with simple virtues, speed and manoeuvrability.