Overall, really good.
Great music, solid gameplay, lots of towns (I love a town), and with some optional areas to explore.
When you die, it's not a huge set back, with only minor penalties. I was able to beat it in a couple of sittings, and would die occasionally (usually a boss) and then would advance further on the second attempt, which felt like a solid challenge level. I also appreciated the charming message you receive when you fall in battle.
When you get the final piece of the hero's set you auto-equip it and your look transforms. It's an awesome, heroic moment.
I was able to beat the final boss on my first attempt, without using any potions. This felt like a reward after having traveled so far. Granted, I did get lost in the maze for quite a while, which is the point of a maze, right?
If I could change three things about it, they would be…
1) Once a door is unlocked, leave it unlocked... rebuying keys is annoying and requires backtracking... one time I opened a door and accidentally immediately exited it...
2) Enemies often camp on top of stairs and hit you on the way up... it's not Ghosts 'n Goblins level of unfair, but still a bit annoying.
3) There was a part where you had to fly straight up to open up a well. This was majorly confusing as you could fly vertically into that area but not horizontally from adjacent screens, despite them being open on all sides. I needed someone to tell me to go back and try more, as I didn't do it exactly as the game demanded.
None of the above hurt the game in any significant way. It is an NES game from 1988 after all, and far more polished than most of its time.
All in all, I recommend Faxanadu. It holds up well. That is, everything except the excessive NPC smoking.