I and some friends have been playing Gloomhaven for about a year and a half now, and we love it. It's unfair to compare it to an RPG - the closest competitors are dungeon-crawler board games like Descent or Star Wars: Imperial Assault.
And I have to say, I find Gloomhaven much more interesting than Imperial Assault. For one thing, it's a lot bigger. We've played a little over 50 scenarios so far and we feel we're getting close-ish to the end, maybe 10 scenarios more (plus possibly some interesting side quests). As a comparison, an Imperial Assault campaign is about 10 missions long (though each campaign has about twice as many as that, because most have options at each stage of the campaign). The game play is very different - Gloomhaven is more of a Eurogame where you have to balance the use of your abilities because you usually have to wait a while before using them again, and IA is more about assigning costs and balancing things that way. You also have characters coming and going throughout the campaign as they fulfill their own goals and retire, only to be replaced by something new.
But yeah, you really do need a stable group of people with which to play in order to enjoy Gloomhaven. When we're playing, we're a core group of 3 with a fourth player being able to join every now and then (fortunately, the game scales fairly well with both the number of players and the levels of their characters).
But sure, you also have dungeon crawlers that are more short-term, like Arcadia Quest or the D&D-based ones. Gloomhaven is definitely a lot more complex than either of those.
To the OP: there is quite a bit of character customization once you start leveling your characters up. Each time you level, you get to choose one card of the two available at that level (or from those you didn't pick at earlier levels) to add to your repertoire, and before each scenario you choose which cards to have in your hand for that scenario. Leveling up and completing battle goals also award you perks which you use to customize/improve your attack modifier deck. You also get to spend money on gear, which gives you additional abilities, and eventually on enhancements which are permanent improvements on your cards.
In addition, there are quite a few characters from which to choose, each of which plays in a rather different way. The starting six are all rather straight-forward, but the ones you add once people start unlocking new boxes can be quite odd. So while the customization of each character is limited (but it definitely exists), the main difference is in what cards the different characters have available at all. A Cragheart plays very differently from a Scoundrel.
