My Baja GPS info is from the Motorcycle side, so finding and navigating all the intricate back road networks. It used to be that BiciMapas (link below) was the best for Baja GPS, the Topo version had everything (except autoroute). Now it can do some autorouting, but still kind-a crap. It's also quite out of date these days.
My last few visits, I've simply used the Mexico maps from Garmin, and they have just about everything you need. But if you are looking for more than maps and landmarks (like POI's / food / gas etc etc) there is no off the shelf software that has up to date info, that info comes from trolling all the GPS forums and picking out pieces from other peoples data.
- Good, but now I personally think the garmin version is a lot better. It's also way over priced, and now locked to the GPS like Garmin maps.
Some versions of US City Navigator include Mexico, and there is also a Mexico City Navigator.
So personally, I'd stick to Garmin's version these days.