Something not brought up a lot is that Enemy Within adds a lot more unique story missions which breaks up the tedium of tons of abduction missions which helps a ton. They also added more second wave options, one of which randomizes the skills for all your units which makes everyone completely unique. It's pretty much on all the time when I play, nothing like getting a heavy with sprint and low profile to wreck everything. And there's some nice options like being able to change languages for each unit, so your badass russian heavy actually speaks russian, and there's the ability to turn off voice tutorials now so no ranting about how using explosives is bad.
Genetic abilities are pretty well balanced and very nice (although mimetic skin is a bit OP.) Mechs are too good though. They keep their aim (no idea why they thought it was a good idea) so you can augment a colonel sniper to get a mech with 100 aim plus all the bonuses they get from skills. They could definitely use some balancing.
Exalt is rather underwhelming, they always seem to be a step behind you. When you're running around with laser weapons, the still have starting ones; when you have colonels and full level mechs, they just start getting basic gene abilities. And they never move to anything more than laser weapons and the first armor so they stay at 9 health which is one shot material once you get plasma. They really need to be a lot more threatening, maybe give them their own mechs, let them get plasma, have them use abilities more, etc. Seekers are pretty useless (mostly because the ai is stupid and goes for front units instead of the sniper hiding in the back). Mechtoids are awesome though and very dangerous, can definitely one shot a unit if they get a chance.
Overall I'd still say it's worth it. The additions may make it seem like the game is too easy but unknown was rather impossible on higher difficulties anyways, enemy within on impossible ironman is still gonna probably kick your butt, and there's even more second wave options than before so if you're feeling really sadistic turn those on too. It's definitely a good buy although try to get it around $15 if you can, that $40 price or whatever they started at was silly.