I found single player MAG really boring.
Same with Warhawk.
I played GTA4 all the way through single before multi, but then found the multiplayer a little sluggish. Games like Burnout are essential to play single-player (to get more varied cars for playing in multi), and both modes are really fun in their own way, but the multi really shines when you're on with mates.
Only other game I have that's both single/multi is AvP - but MAG kinda got in the way of me playing it in either mode
edit: I forgot about Resistance:Fall of Man. I never finished the single player campaign. I got about half way through it before I started tinkering with multiplayer - and the latter was much more fun IMHO.