Blizzards nifty peer to peer patch downloading system that you are forced to use sucks.
I'm getting a whopping 33KB/s download rate (kinda - it goes back and forth from that to 0KB/s). By comparison I can usually pull 400-600KB/s off of fast sites or Steam.
Don't bother trying to ask Blizzard about it either. You get the usual crap: check your firewall/router, disable background apps yada yada. The final tech support answer is, "Go to our forums and ask someone". Grrr.
It appears that maybe one in say 50 connections are actually transferring data. The connections that appear to be working well are pretty much useless since once their "chunk" of data is done transferring they disconnect rather than pickup a new one.
My upload is pretty crappy too. It's pulling like 24KB/s. I don't remember off the top of my head what my upload cap is at my ISP, but that ain't it.
So in summary:
This thing is a chunk of sh1t. I generally like Blizzard but I'm going to be good and irritated by the time my game starts. Hopefully they've done their usual quality job on the game so I'll forget about this.