Finished the campaign about an hour ago or so. Would have liked it to be two or three missions, since it is an expansion and not "just" a DLC, but oh well. Not going to complain too hard. 
As for the campaign, I liked it because it was a bit different than the other ones. Mainly that one needs to hire mercenaries to really have a chance at winning. Unless one is an uber-talented player, I suppose. Or if one wants to grind out a non-merc victory "just because". So for showing off the new mechanic of mercs, it's aces. And it kinda felt good not to be the all powerful death fleet of the first campaigns (at least not at the beginning), nor the ever growing zombie tide of the Snathi one.
Also liked the little surprises I got in it. No spoilers, but I'll just say that enjoyed the scenario triggers where I wasn't expecting them to appear.
The mercs themselves are interesting. Liked the fact that there can be multiple bases on a map. DO NOT like the fact that mercs spawn at the capital of a faction. If only because I know it will be a PITA on larger maps.
I know, I know. Vast majority of the player pool play on small sized maps, thus the mechanic will work just fine there. Still am going to grumble a bit tho for those of us who play on larger maps. ;) If the spawn point for mercs could be modded one day and/or be a user setting (appear at homeworld/merc base/rally point), that'd be fantastic. But I'm guessing that day won't come soon. 
Finally, I liked that the mercs actually made me want to collect/stash money in the early and mid game. My personal play style is to immediately pour all of my money I get into upgrades, trades and improvement rushes, when needed, and not bother trying to get large amounts of cash until I am into the late stages of the Age of War. I know that many people are just tripping over money in the way they play the game, but for me it actually makes me want to store up a bit. Perhaps even change my play style a bit so I have more money on hand during the mid-game.
So job well done on that score. 
Overall, despite what minor grumbles I might have, I think it is a very good expansion. Haven't even sunk my teeth into it by going into sandbox mode and letting it rip with both the Acreans and the Torians. Should be fun.