11g / USB-2 adapters were a long time coming, but the dam now appears to have broken and product a' plenty from BuffaloTech, NETGEAR, Linksys, D-Link and others is flooding into the market.
BuffaloTech's WLI-USB-G54 performs pretty much like its notebook (CardBus) adapter, but also gives you the option of attaching an external antenna. Just as with 11b USB adapters, pricing is higher than its CardBus equivalent, but not as high as using a wireless Ethernet adapter.
If you've been waiting for these beasties to be available so that you could get your desktop onto your wireless LAN without having to pop open the case and wrestle a PCI-based board in, then go ahead and take the plunge. At a maximum transfer rate of 480Mbps, USB 2.0 has enough bandwidth headroom to not crimp 802.11g's maximum 54Mbps raw data rate and have plenty left over for other devices connected to it. Pay your money, take your choice and set your desktop machines free!