Im fairly new to backtrack but was using a belkin pci wireless adaptor with not alot of joy, looked at buying an alfa, but the price put me off, anyway had a look round ebay and found this little number
GSKY Realtek RL8187L USB Wireless Network Card Adapter on eBay (end time 11-Jul-10 07:07:51 BST)
Works straight out the box (well it didnt come in one, but you know what i mean), does the lot monitor mode, packet injection and shows the power too.
Took about 5 days to turn up from Hong Kong, which i didnt think was bad at all.
Hope this is of help to someone out there. Chris