I've really enjoyed it aside from the fact that it goes a bit too over the top towards the end. The early parts of the campaign have some cool characters (Alarak is the most entertaining SC2 character of all time) and the general theme of rallying all the protoss under one banner is pretty interesting. I was surprised that story wasn't a copy of Warcraft 3 (although you can notice some similar mission design, I was surprised I didn't have to pick up 10 shadow orb fragments on Ulnar) since it looked like it was going to end up being that from HotS. So, the campaign's really great up to the last few missions on Aiur. I was pretty disappointed that Amon's host body got simply blown up by the Spear of Adun, I was expecting for it to put more of a fight. Then the last mission was kind of a copy of the one of the first missions where you had to defend a xel'naga temple. Overall though, the new units were a lot of fun to use and the story was decent.
Now, the biggest problem which I had was the epilogue. When you start the first mission you see a gigantic squid. My reaction is "Oh, that must be some kind of a mega hybrid made by Amon" but soon you realize that it's actually Narud who is apparently a Xel'naga now. My question is, if you wanted to prevent Amon from transferring himself into his Host Body so much, why was Narud in HotS so weak and why didn't he pick some kind of a more impressive host body than just a human scientist? Aren't Xel'naga powerful enough to destory all the races and recreate them again to keep up the infinite cycle? But whatever, you a-move a bunch of void rays and destroy him in a few seconds after going through all his defences.
Then you meet the last Xel'naga Ouros who wants to transfer his essence into Sarah in order for her to become a new Xel'naga and defeat Amon. Now, my first disappointment was that the Xel'naga are literally all just giant floating squid people which I wasn't too sure what to think about. The main problem here is that the prophecy was about two races of purity of form and purity would combine into a new Xel'naga not about Keriggan combining getting Xel'naga's power. I was totally expecting Artanis and Keriggan to just combine into a Zerg-Protoss Archon Xel'naga thingy. So now, you defend Sarah for a bit and then she becomes a fire angel thingy. I'd just like to remind you that before in the story she went from human to zerg, became a human again, turned into a zerg yet again and now she's a angel thingy. Also, why is she the only Xel'naga which isn't a horrible tentacle monster?
But whatever, now God-Keriggan goes to battle Amon who's also a tentacle monster and lasers the hell out of him. Then it cuts to 2 years later where normal human Sarah visits Jim again. Is she still a Xel'naga? Why doesn't she need a host body like all the other Xel'nagas and how did she get out of the void?
So yeah, the story ends up being a bit weird but the gameplay's great.