Good morning, Gargamel. I don't know anyone who has used it for UC, but I do know it is used to treat certain cancers, rheumatoid arthritis, psoriasis and sometimes Crohn's. It works by suppressing the fast-growing cells in your body, i.e. skin cells, bone marrow, cancer cells. My experience with it was largely with cancer. I know it can also have some nasty side effects, which I'm sure you've already looked up.
Also, MTX is a very potent cause of serious birth defects if either parent is taking it when a pregnancy is conceived, so you have to use effective birth control while on it.
This link provides a little more information about
MTX and other drugs we sometimes take:
http://www.gi.org/patients/gihealth/ibd.asp
I did a little searching and did find a study from the UK showing that it may be effective:
http://www.ncbi.nlm.nih.gov/pubmed/15709988
Hope this is helpful to you.