Playing the campaigns is decent entertainment, the first time around--kinda easy (on normal mode), finished it in 3 days (1 day/campaign).
The post-human campaign movie is well done (Death Knight walking into throne room) but the later ones are not nearly as good.
Multiplayerwise, expect MAJOR lag when there are too many units on the map.
No more 4v4 or even 3v3 battles, even though the game supports 6v6--it is unrealistic--it will play like 1 fps. You can test this simply by playing vs multiple computers in single player mode.
You might get decent 2v2 gameplay, if everyone is using a fast 1 GHz+ processor on broadband and if one team attacks early before both teams max out on fighting units (realistically about 25-35 units per team). So, expect teams to quickly gang up early and kill players just to keep the game less laggy. So much for the no rush element mentality. As you will see, this is much disappointment when compared to the fast paced, multi-allies gameplay of Starcraft/Broodwar which is arguably the best RTS.
3D units for an RTS was a major error. The whole concept of RTS requires the ability to see the majority of your troops and to quickly control of units from a top down perspective, making 3D engine unnecessary. This ain't a first person shooter for crying out loud. Also, the 3D character actors look blocky and ridiculous when compared with the CG movies.
Without decent multiplayer speed, you gotta ask yourself, is a single player game worth $59? After finishing it, I have to say not quite.