I have a Alfa AWUS036H and it works with the rtl8187 driver. I haven't tried packet injection. Some of the third party antennas have gotten mixed reviews. I'm using the stock antenna and the signal is good.
My main gripe with it is that the automatic rate control is a bit poor. It has a tendency to default to 54Mbps, and from time to time will completely lose the signal unless I set a lower rate using 'sudo iwconfig wlan1 rate ...'
Also, this is a 2.4GHz b/g radio only. No 5.7 GHz, and no 802.11n.
It can do 1000mW (30 dBm) but you will need a powered USB hub as this draws a lot of power. Also if using CRDA you will need to set a country code that allows 1000mW (eg US, BZ, NZ).