Generally speaking, the maneuver isn't as effective as you'd probably like. Against certain teams and in certain situations it can be absolutely brutalally effective but most of the time you're almost certainly better off simply shooting people instead. One particular game stands out where most of the opposing team on a groundwar game (Domination, Terminal) was using some variation of the witch build. They were doing well early on and then it dawned on me that the solution was simply camp and lock off choke points and gun them down like a pack of L4D zombies. In another instance, in Skidrow, a team used a similar strategy and managed to utterly annihilate our team.
Basically, if you have lots of cover and close contacts, the witch build is excellent. In any other case, it's probably a terrible, terrible move.