Im doing the same thing you are right now but for wedding receptions, usually very low light as well.
Ive got my elinchrom skyport triggering through pc jack on the side of my 40d and my 580ex ii not firing using only for AF assist on my camera body shoe, it works just the way I need it to.
I will definitely be trying the st-e2 yongnuo knock off when I can get ahold of one. If that doesn't work Ill cave and buy the official canon version.
If you aren't concerned about size/cost you might consider a 580 instead of st-e2 since the cost is not much higher.
Line of sight: The flashes communicate via IR, depending on ambient lighting and the other types of lights/flashes/party lights filling the room this can be affected. In a small room you can bounce the IR signal, similiar to how you can change the channel on the TV without aiming the remote directly at the TV (typical American analogy) I would recommend using your cactus AND an AF assist device.
Edit: eBay market on ST-E2 is pretty low right now.