Hi Angbur,
I went to Glaorgan Uni, the whole of Treforest is pretty run down to be honest *praying im not offending anyone*. I wouldn't say there were any really bad areas, it is very safe and a nice place to live/study, but there are no areas that really spring out as particularly nice areas. The whole of Treforrest is geared towards students, so all of the housing is very cheap, and the accomodation is all really geared towards students, hundreds of terraced houses.
I do really miss the Pink Shop tho!
There are some nice houses on Llantwit Road, away from the university (towards the left), this end is quieter than the main student area, and the houses are big.
Don't live anyhwere near the railway bridge right next to the university. I lived on John Street and we were flooded 3 times in 2 years, the river Taff burst quite a lot, and this area always gets very flooded. Especially Park St and some areas of Forest Road.
As for the Uni itself, it was the best 3 years of my life, enjoy it if you go!!
Good luck.