My recommendation: If you're not ultra Starcraft fan, rather waiting until the price drops. Because so much is the add-on is not ago that it justifies the price. The multiplayer mode of StarCraft 2 - Wings of Liberty is still supported and there are enough players online, so even here you have to worry about.
2 stars get the add-on but as the campaign can be played nice. The story seems to be ok. In multiplayer mode there for each race a few new units, but I still could not fully test their effects.
All in all I feel that the purchase of nearly 40 has not really paid off.
