I can't say I was disappointed, because I liked it at first, but Gloomhaven really became a drag after a year or so of playing. I feel like you really need to be invested in the lore and story to get anything out of it after a while, otherwise it's basically glorified, over-complicated chess. It doesn't help that 90% of scenarios have the same winning condition: "kill all monsters". I feel like there could have been a lot more depth to the actual gameplay, and not just the fluff in-between. What's more, each scenario takes 2-3 hours at best, and to make any real progress you need to set aside at least 6 hours per session, which is crazy. It's basically a job at that point.
Also, in the later stages, when you have a level 3-4 party with unlocked classes, encounters become exhausting, because you need to keep track of a million modifiers and buffs/debuffs, sometimes cancelling out eachother twice. And it's not a Gloomhaven session if you don't keep going back to the BGG forums for rule clarifications. It's a mess of a game, really.