I borrowed a D-Link WUA-2340 USB and it was horrible. It got good signal on my old P4 Computer (USB 1.1) and when I switched it to my computer I build for my Dad with Vista and USB 2.0 slots, horrible. I was always at 2 bar's according to the Vista wireless manager and the D-Link software doesn't really like Vista. If one day overheated and told my Dad it burnt out. Told him to go to microcenter and buy a PCI one, didn't specify on brand, and he got a completely no name brand. Put it in the computer, installed the drivers, and 5 bar's in Vista.
Considering the PCI card was only $20 its great, and I get great speeds off it. My main computer that is connected to Ethernet to the modem gets 20K download and 4K upload on speedtest,net and my Dad's computer gets 15K download and 3K upload.
For me, I will only go with PCI cards for desktops and leave the USB's for laptops.