I just got 2012 and found it was much more than an "expansion pack". The graphics are outstanding. I run a 36" HDTV as my monitor and with 2010 it was shimmering and had a lot of sawtoothing. 2012 is like watching HDTV, so crisp and clear and the finish on 2012 rolling stock is great. I also found that I can load a railyard with a large amount of rollingstock and no lag.
I am also surprised with the amount of old content I have been able to import with minor changes that runs well and looks great.
I dont have any use for MP but I think the purchase was well worthwhile.
cheers,
Mike