a) Yes
b) Incredibly so.
Let me quote you the praise it's gotten:
"Takes XCOM to a new level." - Jake Solomon, XCOM lead designer
"We're basically a 20-hour tutorial for Long War, and that's okay." - Solomon
That's the level of regard it has from the designers of XCOM. It is basically a mod that is on the level of an expansion pack. It makes the entire game more difficult, but it improves dramatically on nearly every aspect. More soldier classes, better balance, an expanded tech tree, an officer system, and UI/QoL improvements to just name a few things. I played one game of XCOM all the way through and now I'm playing one with The Long War, and it is just so much better.
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You can download it
here, although you have to register for their forum first. It looks like you have to pay to register, but you actually don't; it's purposefully misleading that way. Just don't click any of the pay options and hit register and it goes on like a normal free send-an-e-mail registration.