In my 5-Minute Dungeon review, I mentioned this expansion improves the game by adding variety and strategy. It's not quite enough to make me want to play, but it does make the prospect of being roped in a little less annoying.
What does it change? The headliner is curses, which are boss-specific cards shuffled into the dungeon deck. Curses restrict all players in some way until they are cleared. For instance, the first boss, the Baby Barbarian, has a cursed blanket which restricts players to using one hand. Curses stack, which means you can find yourself hampered in several ways. If someone inadvertently breaks a curse restriction, they must toss their hand into a (metaphorical) void where they can't be recovered.
I don't think I'd care for curses except they replace the ordinary symbol-matching door cards. So they don't necessarily make dungeons harder. The expansion also introduces a sixth player who specializes in removing curses. The Druid is particularly useful since her ability moves curses to the bottom of the deck so they can be dealt with just before the boss encounter. There are also 10 other action cards added to the other five character decks, which makes them a touch stronger and more interesting.
The base game plays best with all 5 players because each deck brings a unique strength to the table. With fewer then five, that means one or more deck isn't in play. Without a huntress/ranger, you won't have as many arrow cards, for instance. The new black deck helps by including infinity cards that count for any number of specific symbol. Be sure to save infinite arrows for when you need them.
The expansion also introduces artifacts, which are one-time-use cards with special powers. It's easy to overlook (at least for us) but anyone can use any artifact and you only include artifacts from player decks you aren't using. (We mistakenly give the artifact to the associated player if we aren't careful.) It's a method of balancing the team so you don't have to worry quite so much about missing a special ability.
Overall, this expansion improves the game without solving the problems of the game system. Every additional card increases the variety of the game, which is welcome. Boss-specific cards make planning before the timer starts a more productive activity. But the core of the game remains matching symbols as quickly as possible. That puzzle just isn't enjoyable for me.
If you have a group that enjoys 5-Minute Dungeon and you don't have the Curses expansion, I can't think of any reason not to buy it. If you don't like 5-Minute Dungeon, the expansion won't likely change that.