AoE knockbacks are horrible to use on teams. They scatter herded enemies so that nobody else can use an AoE attack effectively.
Power Siphon accepts recharge, endurance reduction, and to-hit. Like most build-up powers, it is best to slot it with 2-3 recharge IOs. Some builds may put sets in it for a set bonus, though.
Try searching the forums for builds.
I plan on making a KM/WP brute. I have questions being that this is my first one. Starting with, Repulsing blast, Is it any good? Personaly i hate KB but i love aoe damage, even if it is a cone... Two, how should i slot siphon power? Im trying to slot for alot of recharge with out gimping my damage or defence/resistance/healing on WP. Someone who has built a successful one reply, I talked to someone and there reply in game was "I took all the powers, hes a blaster tank and is awsome" <-- this made me go o(-.-)==o)x_x)