I played Enemy Unknown to hell and back so this was an easy buy for me.
Just finished my first full playthrough. Played it on normal so I could explore and experiment without risk, I'm gonna go again on classic now. My thoughts after finishing (Wall of Text ahead):
The new enemies are nice, it's great that they found a way to keep sectoids semi-relevant in the later months and the seekers are pretty scary early on, trying to pick off isolated squad members.
MEC troopers are great fun, but don't feel as useful as a regular soldier, I'd rank them above a SHIV but below a major/colonel of another class. Even with all their tankiness (my heavy MEC ended with 30hp) they seem to be too ineffective at range, my MEC missed about 65% of her shots (though that could just be Hidden Potential boning her out of stats) and their weapons run empty far too quickly. In melee they're a different story entirely, and the kinetic strike alone is enough to consider bringing a MEC along.
Gene mods... I didn't use them as much as I could have, partly because I don't usually have 15 days of downtime to give my A-Team soldiers so they can get modded. More than that though the gene soldier armor is atrociously bad looking and I couldn't bring myself to have all my people running around with those stupid sleeveless armor pieces. On the soldiers I did mod though the mods themselves seemed pretty nice. I especially love the leg mod that lets you jump high and the mod that gives a bonus when you kill things.
I've got mixed feelings about Training Roulette. On the one hand it let me get a super assault with sprinter and the jumper mod who could cover half the map and still unleash 30 or more damage on things. On the other hand I had several heavies who had savior and the triple medkit ability.
On the ability changes, I'm happy with most of them. I now think that snapshot is superior to squadsight because of the reduction in aim penalty and the no-crits on squadsight. I had a snapshot/in the zone sniper who was taking out multiple enemies per turn during the base defense. Close & Personal is amazing, but I think it's further unbalanced the classes. Assaults were already premier damage dealers late game and now they can move AND put 3 shots on a single target in one turn. Covering fire is still, in my opinion, hot garbage. One of my uber assaults got whacked with covering fire in the major slot and used it precisely once. Overwatch is great for stopping people from moving around the place, especially if they're coming from out of sight, but if I wasn't comfortable with shooting at something before, why would I want to take a shot at -20 aim? I'll admit this could just be my playstyle, I prefer hunker down to overwatch in a lot of cases.
I didn't really use many of the new items. Flashbangs were great against thin men early on, and midgame pre-titan respirator implants were stock standard for everyone except the medic. I don't like the idea of the needle or gas grenades, and I don't use stealth so the ghost grenade wasn't important to me. I can't really see the point of the mimic beacon and I was starting to pump out lasers by the time I got reaper rounds.
I was initially a bit disappointed by the new voice languages. I was hoping that there'd be more variation (I.E british/canadian/australian for english), but I'm not really bothered by it. It was nice to have soldiers speaking languages appropriate to their country, even if I couldn't understand 80% of what they were saying.
EXALT. I'm not really sure what to say about EXALT. They were dished up as the big new threat but they never really materialised as such in my game. I ran one, maybe two scans per month and did the covert mission and they never once did anything outside of the scripted action they take when they're first mentioned. Eventually I just raided and destroyed their base with little effort since by that time I was toting plasma and they still had lasers.
The base defence mission was great fun, if a little protracted. The first two waves were the hardest for me since they drop a whole bunch of units on you while you're out of position, and the cyberdisk wave was much larger than I expected, but after those I kinda rolled through it and finished with no casualties since it gave me my 4 biggest badasses and some rookie grenade launchers for backup. I'm guessing it'll be harder on classic.
The Progeny missions were boring, barely worth my time. Slingshot was interesting because it had some new mission types in it and at the end you were rewarded with some badass technology. Progeny has a stock standard VIP recovery and two timed 'reach the end' missions. In the case of the latter two they were barely even a challenge and I blasted through the last one so fast I thought I had missed half of it. And at the end of all this you're given four low-medium rank untested but guaranteed psi soldiers.
The Newfoundland mission, man that was scary on my test run (I played a quick slog before doing the full playthough), I got it the middle of month 2 just as I was getting laser rifles out. Hoooo boy it was close. On my full run I got it about 3 weeks before I did the temple ship so it wasn't really much of a challenge.
Quick notes:
I didn't experience a single instance of the teleport bug which is nice since I always used to have that at least once per playthrough in EU.
I like most of the new maps, I only got the narrow rainy highway of doom map once and didn't get the teleport-glitch prone graveyard at all.
Lead By Example (the new OTS upgrade) is spectacular, my 135 (165 with psi armor) colonel turned even chucklenut rookies into psionic resisters with it.
The new armor customisation is cool, I'm undecided on the weapon coloring though.
Sectopods are much more tanky now, perhaps a bit too much so.
Finally, medals. They're a nice little bonus but not really much of a big deal.
That's pretty much all I can think of. I like EW and it's about what I expected, definitely worth the price of admission. I'm off to start again on classic.