I had purchased the Miniature Wifi (802.11b/g/n) from adafruit for my raspberrypi, but I ended up having serious strange problems with that part and ended up returning it for a refund. Since this was based on a RealTek chipset I hastily decide to avoid anything RealTek based.
I then got the WiPi module from newark, which was based off a RALINK chip and claimed to be compatible by the seller. This worked great initially but would randomly disconnect if the connection got idle for too long and had trouble reconnecting. I ended up messing up my SD card FS twice because of these disconnection. Searching around this other people were facing the same problem.
More searching revealed that people were having better luck with the RealTek based AirLink101 USB adapters. So I then got an AirLink101 5099 from amazon, which has been working great for the last few days.
I am now seriously considering returning the WiPi back to Newark.
If you are considering buying a USB wireless adapter my suggestion would be to get one of the AirLink101 Realteks.
More information here: http://elinux.org/RPi_USB_Wi-Fi_Adapters