My doctor just gave me this option last week as well, when I had to stop Imuran because it was affecting my pancreas. He told me I'd take 7 pills once a week according to my weight apparently. He said mostly used for Crohn's patients, he's had alot of patients with UC who can't use Imuran/6mp use it with success.
At the time I was doing pretty well on 10mg of Prednisone and decided I wanted to try weaning off the Prednisone further before I went to the Methotrexate. I take Colazal and Mesalamine enemas too however. I'm at 7.5mg of Prednisone now though and starting to feel a little off. Still normal BMs some of the time but sometimes not. No blood (although I tend to not get that as much at the start of a flare). So I may end up going on Methotrexate - I am going to wait to see how I do over the weekend. If I get worse I"ll be calling on Monday to start up on it.
Did your GI say how much you would need to take? Sounds like it can make you nauseas the day after you take it - my GI said most people take it Sunday night as they don't care if they feel sick going to work on Monday anyway :) He did say he could give me Zofran for nausea, but that tends to give me a migraine so I doubt I'd take him up on that offer.