I have used Streets & Trips 2009 in Mexico a lot. I've used it in the following states: Tamaulipas, Veracruz, Coahuila, Chihuahua, Sonora, Sinaloa, and Durango. Generally I'd say it has very good coverage from major roads down to small pueblos.
Perhaps the biggest limitation is when you try to map along the autopistas, especially the newer ones. For instance, you can't map a trip from Tampico to Veracruz using the autopista from south of Tuxpan, then bypassing Poxa Rica/Papantla (180D, I think). Of course, this is a common problem for all Mexico maps; the latest Guia Roji atlas doesn't show some autopistas that have been done for years (Ojinaga to Aldama, for instance).
The other limitation is obvious- roads in rural areas are iffy. We travel in very remote parts of Mexico and its utility is limited. For that I depend on the digital versions of INEGI's 1:250,000 topos for the whole republic. You can easily load these gif or tif files on a laptop, or print out sections you need. If you can get the CDs directly from INEGI, they are reasonably priced.
The GPS feature has worked well, but usually it locates me in a big yellow area with nothing around. Of course, that can mean we're in a beautiful, remote place with no one around with great birds, plants, and scenery to enjoy!
As with other similar programs, projected driving times in Mexico are more fantasy than anything. But I do find it very useful for location finding (manually scanning) and trip planning. I've checked the internet for other options, but so far I think S&T 2009 and my INEGI topos (with some help from Guia Roji) are best.