I have no first-hand knowledge. That said, the madwifi driver was heavily contributed to by Sam Leffler, who is an extremely clueful BSD guy. If he worked on it, it's likely to really work. The ar5k driver quite explicitly says it's not working yet, so I would interpret that as... it's not working yet. I think if you start with the madwifi driver and keep an eye on the ar5k guys, you'll get the best of both worlds.