Over at Mobile Pipeline, I've reviewed the HWL2 from Hawking: I'm not very pleased. It's a good high-gain USB adapter, but its Wi-Fi detection function doesn't offer anything that several newer, better devices have, and its software for control is terrible. I prefer the RangeMax USB 2.0 adapter from NetGear that's $10 to $20 more, but it's just a USB Wi-Fi adapter and its Video54 technology for beamforming works quite well in picking up distant 802.11b and 802.11g signals.
Why use high gain when you can use smart tech? Switching to a higher-gain antenna means that you're letting a weak receiver or transmitter listen more carefully. Using beamforming and MIMO means that you're letting multiple antennas distinguish a signal more clearly or form a signal more coherently.