I have an HP DM4 laptop running Ubuntu 13.04 (although i've noticed this on 12.04 and 12.10). When I am connected to the WiFi at home none of my housemates are able to get decent connection. They can connect but not use the internet. I've noticed it a number of other places including hotspots in motels etc.
It's not a problem for me but I am becoming increasingly unpopular at home!!
I have the Broadcom STA Wireless driver installed and am using it.
Let me know if I can give any more useful information.