I played the MP Free Weekend and was so underwhelmed by the terrible guns, and turned WAY off by the lack of mod support or dedicated servers.
The "Create-A-Class" in the SP caught my eye, though. Would this be worth just playing the SP & ZM? Also, I wouldn't buy any expansion packs because lolno, not doing THAT again. --- http://i.imgur.com/LJQ7UFC.gif
The create-a-class is strong in MP as well but the SP campaign is only about 10 hours long and I've never found it particularly replayable. I can't comment on Zombies, as I find it terribly dull. :) --- Must try harder