Initial tests show a much higher RX sensitivity than most cards, plus the availability of 2 antenna inputs makes it nice for semi-permanent install into vehicles. I personally have 2 directionals pointed out either side of my vehicle fed into the router.
The other nice thing is that since it's reporting it's data over cat5, you can install the router very close to your antennas minimizing RF cable loss. You could even just install the router on the roof of your vehicle so long as you can get power and cat 5 up to it (POE anyone :) ).
They can also be daisy chained together, since each unit also is a switch, you can just network all of them together for as many IP addresses as you think you can subnet.
The Sveasoft firmware is a wonderful thing to play with. It's additional features are something useful to anyone using these units at home and just want to play a bit with them as a Kismet source.
OpenWRT is superior in many ways since it's a much more open architecture, you can make it do what you want, rather than what Sveasoft gives you. The inclusion of the ability to store files on the router through power cycles makes OpenWRT alot more useful for more permanent uses such as running the Kismet_drone for mobile or stationary monitoring (i.e. cheap IDS monitor)
One final note. I've seen various reports around the web about people running the full kismet_server on the router. While not providing much storage space for logs, it's an intriguing idea, but not very well documented and in practice is probobly pretty useless.At least one person has done this according to this post. If you can provide more info, please do in the interests of completeness of this guide.
Lerdorf's Toy page about Kismet on the WRT54G. Much of this How-To was
drawn from there
Netstumbler forums post by Troycicle (Banned from the forums BTW) that helped as well
Seattlewireless.net's resource page on the routers. Lots O' info
Void Main's Linksys WRT54G revival guide - Very useful if you brick your router
OpenWRT.org - Lots of documentation and instructions there. The forums helped solve alot of problems and provided alot of info