Well, what Gameboy pointed out is the primary reason why it is a bit of a mess. There are advantages and disadvantages to open source, but it'd be better if at the very least the developers released their OWN binaries, so we don't have to rely on others (like Nic, as MTX says) to do it for us.
Anyway, I use Divx Pro when I have to get the maximum out of something. With all of the settings jacked up, it is very very slow, but I get excellent quality. If I have a small target size, I'll use Divx and take the speed hit. I always use Divx if my source is interlaced, as with Divx you can just *leave* it interlaced and let the decoder handle that if necessary. If speed is a concern and/or if my target filesize is a bit larger, I'll go ahead and use Xvid. Also, Xvid is totally free, whereas with Divx only the standard version is totally free. Divx Pro costs money, unless you use the adware version, which most people are unwilling to do. I personally don't think $20 is bad if you use it all the time.
So they both have their ups and downs, it is best to have both if you can. I have no experience with single-pass encodes, either. I always just decide what bitrate/filesize I want and do 2 or 3 passes. Oh yes, I did forget to mention this before - Divx (Pro) really shines with live action, with anime Xvid is just as good unless you're a freak like me and tolerate ridiculously low fps.