I played through it and got all achievements.
The remaster is pretty damn impressive and I really like how the improved the graphics, the general gameplay (the improved AI you mentioned) and that you do not have to jump through hoops to play it and they restored the amazing sound track.
It is probably much harder for people that are used to the Call Of Duty style of shooters these days. No regenerating health, no autosaves for the most part.
You can however just save free whenever you want.
I played Quake since 1998 or so and I can't really judge the difficulty of much things fairly.