I like Hat Yai a lot, it is a great city, but others have different opinions. It has a great downtown shopping area where you can get just about anything. As well as having all a McD, Pizza Hut, Dairy Queen, KFC, etc, they have a Sizzler. Loads of Malaysian tourists go to hat Yai for the weekend so it has a decent nightlife. Hat Yai is a little on the dirty side, but no worse than many Thai cities. A good number of westerners around Hat Yai and you are less than an hour by bus away from the beach, it is only 25 km, but you know buses in Thailand. I never had a problem with any bad atittudes towards foreigners, in the downtown area there are places that overcharge tourist, but locals can easily avoid them. One thing some have a problem with is a large number of Muslims. I don't have a problem with Hat Yai musims, but some people have an unreasonable fear of them.
I have known a couple of people who worked at the Medical School and they didn't seem to have any complaints at all. The hospital itself sucks, (they have a written policy of charging foreigners double the amount they charge Thais), but the medical school seems alright.
Any more questions, just pop away, I did a 1 year stint in Hat Yai.