I just did a trip from Bratislava to Leipzig with MP mod. The cool thing is that you can continue your "career" from SP.
Admins are around as I saw messages saying player x is kicked/banned for blocking/ramming etc.. I didn't have any problems with that.
I'd say that if you have anyone who you can play with it's OK, otherwise stick to SP as there is no traffic and trucks are passing you from time to time so roads are pretty empty. Driving in a convoy would be cool, but oh well I'll stay in SP.
Also I always thought (and still think) it's boring, but the nice part is that I'm playing it almost every evening just before I'm going to sleep. It works perfectly.