I worked at UTM a few years ago. It was my first EFL job, but I was lucky enough to get the job because I had a lot of previous teaching experience in another field. In the beginning it was all a bit confusing, but I found the other teachers (a lot of whom are still there) very friendly and helpful. The resources weren't as plentiful compared with what I have seen since, but there was definitely enough to prepare good classes. There was also a fabulous bookcase filled with English language books (fiction and non-fiction) just for the teachers! Sometimes it was a challenge to get an overhead projector or a computer lab with internet access, but with enough notice it could be done.
Some things I miss about UTM now that I'm doing the private language school thing are: my own office and computer, a working environment where the students' education was a lot more important than making money and having enough time to actually think about my development as a teacher while at work. It was great to be able to spend time studying methodology and get paid for it!
On the down side, some people might find it dull to be at work all day, when they are only teaching a few classes, but you do get used to it. I also found it difficult to watch the students working so hard to get their degrees, especially after the large amounts of lounging around I did when doing mine! Sometimes it seemed that there were a lot of rules for the students, but then once I got to know a few graduating students I realised that the rules helped them learn a lot of really good stuff that would definitely prepare them for their working lives. A lot of the students came from families that had never had someone go to university before, and it was great to see them being successful.
Huajuapan de Leon is an interesting little town. I haven't been there for about a year and a half, and haven't lived there for almost 3 (where has the time gone???

) but while it was a little quiet at times, there was always something to do. I am very used to living in small towns however, so I didn't have much trouble with that side of things. For a touch of city, you can jump in a van and go to Oaxaca for the day or the weekend - its about 2 1/2 hours away. There are also some great old monasteries in the area, and all you need to do is jump in a different van to go to see them, or the Huajuapan museum often organises tours on weekends. I used to like going to the museum to watch art films on the weekends, and there is also a cinema in the centre of town that shows newer films, or you can always put that on your list of things to do in Oaxaca. I'm partial to the odd beer, and there were plenty of places to go with a nice atmosphere around the tree filled main square.
Also, with the amount you get paid, it is possible to go away for the long weekends and vacations. Mexico City is 6 1/2 hours away, and I went there a couple of times, as well as having a beachy Christmas in Cancun and a trip to the Copper Canyon (in the north of Mexico) at Easter, and some other places as well. When I left, I had about $2000 US saved up.
To sum up, working for UTM is pretty good, although probably not for people who are dedicated big city folk, and living in Huajuapan offers a good way of life, and opportunity to get to know a side of Mexico a lot of people would never get the chance to see.