I loved OMD, but the sequel really didn't work well for me. The first one gives you levels that work in YOUR favor. The second one gives you open-ended bullshit that works in the enemy's favor, and practically forces you to play co-op. While you CAN finish the campaign by yourself, it's usually very dependent on a particular build with little deviation possible. (as opposed to the first one, which you can finish in dozens of different ways.)
I will say this, though: The endless levels of OMD2 are worth the asking price by themselves.