ICYMI ... a ball "experience" inspired by the Netflix romance drama promised to be a royally good time for fans. That is, until they arrived at the “Detroit Bridgerton Themed Ball” -- walking into a no-frills party ... reminiscent of Glasgow's Willy Wonka pop-up factory.

We spoke with a few of the attendees after the bare-bones affair -- which was not affiliated with the streaming service -- and they dished to TMZ what went down during their underwhelming time at the "Bridgerton" ball.

Katrina from Detroit tells TMZ ... she spent $450 on basic tickets, and an additional $650 on attire, hair and makeup. Yet, when she arrived at the highly anticipated ball, she witnessed an understaffed event ... where there was nothing but long lines for "terrible" backdrops.

There was no one checking tickets at the door, and the promised live entertainment was just a violinist, one pole dancer, and a random guy in non-themed clothing doing the "Cha Cha Slide."

Another attendee tells us she spent $600 total on tickets, a hotel, and apparel. We're told there weren't enough chairs, but there was a ridiculously long line for mocktails.

She said "nothing was on theme" ... a real disappointment for fans of the period drama.

Another participant said she spent $150 per ticket on the ball, only to experience "tacky and cheap" decorations, relating she felt the whole thing was "extremely inappropriate."

Attendees were so angry and let down, word of a possible class action lawsuit has even circulated online.

However, at least one entertainer was happy. Professional aerialist Tink tells TMZ ... she was treated very well by ball organizers, noting she was paid $800 to perform 3 pole sets -- which only took an hour.

Tink says she received a "warm" welcome at the ball ... though, she could tell many were "confused" by her presence. It may've been her routine to "Moves Like Jagger" that threw people off ... but, in her defense, the organizers asked her to perform it.