Isekai Quartet is a crossover between the three best isekai shows and Overlord that takes place in a chibified school setting. It's not a full team up show, as you might see in Western comics, and is instead like the one-off episodes of a sitcom where a few characters from another show visit and make references to their own world. This isn't necessarily a bad thing, just don't go in expecting it.
The worst I can really say about it is that it's less four different isekai casts interacting with each other in a school setting, and more four different isekai casts in a school setting, that occasionally interact. There are things you wish they could have done, but the vastly different tones and power levels of each show makes it nigh impossible to do right and really isn't worth the effort in the 9ish minute chibi format.
Will you enjoy it? It depends. If you have seen at least two of the shows (ignoring Tanya's, as they give you a quick overview right at the beginning), you will likely laugh at a number of the gags that reference their respective shows. If not, I'd say skip it and watch Konosuba/Re:Zero/Youjo Senki and/or read the Overlord LN if isekais are your cup of tea.
