Although I haven't finished my first playthrough yet, I gotta say that the game feels a tad too busy now with all the different threads going on. I really wish they would've revamped or completely dropped the main thread and just made Enemy Within it's own campaign. Doesn't really help much that I've played those same missions a ton of times before and they were probably already my least favorite parts of the game. I guess with the mucn more frequent missions at least it forces you to have an even broader stable of soldiers.
Other than that I wasn't expecting to love the MECs as much as I do. They're just awesome, and the fact that you now have the option to make "useless" soldiers into something else is just great. Not too impressed with the mutations though, they should've really taken them just as far as the MECs and not just some minor bonuses. The medals were a nice little surprise, but lacked some explanation. I guess overall I'm a tad torn on the package. Just really wish it was a tad more of an expansion rather than substantial DLC.