well I still havent done MP - my PC did funny things when I tried to get onto battlenet. I suppose I could have pursued it further, here or with Blizzard, but it wasnt really something that important to me.
Instead ive continued with SP. The Zerg scen where you have to stop the dark templars from escaping was pretty good - you could NOT (at least with my level of skill) simply sit there turtle up. My impression was that you not only had to protect the escape sites, but you really did have to come out and attach the Protoss.
Ive since started the next scen, where you have to take a drone to the crystals.
Seems intermediate - you want to move to the second big resource area relatively quickly, but if you come out too fast (whcih i did the first time i played it) you will get weakened -so, i was too weak when the first carrier arrived. Im not sure if you want to turtle up for a long time, or just somewhat longer.