That all sounds fairly generic and derivative, and this is fact the case. Hidden practitioners of magic in the modern world can be found all over fantasy literature/media. A team of heroes having to learn to work together while saving the world is even more common. But those are familiar tropes for a reason...do them well, and the result is really entertaining. And The Librarians pulls it off.
Mostly this is a adventure-of-the-week series. Each of the four seasons has a Big Bad villain to defeat, but they don't show up every week, and outside the start and end of the season they play a minor role. There's all the usual magical plot suspects - hauntings, regular people gone power-mad, evil masterminds taking towns captive, etc. Villains are often literary or mythical figures, playing off stories we're all familiar with. Again, we see this stuff all the time for a reason...it works when it's implemented well. There were a few episodes where even I had a hard time suspending belief, but for the most part things held together well enough.
Where The Librarians really shines, in my opinion, is the characters (and the actors who portray them, all excellent performances). Initially, the Librarian Flynn and his newly appointed (and very skeptical) Guardian Eve Baird are chasing down an evil mastermind. They're rapidly joined by three other Librarians, each of which have both significant abilities and character flaws, and the immortal Library caretaker Jenkins. Before long, Flynn goes off on his own - he reminds me of The Doctor from Doctor Who, incredibly clever and capable but ridiculously flighty - and the core team of Baird with three inexperienced Librarians and Jenkins is formed.
Every formulaic weekly episode is made entertaining largely by the way that those five characters (six on the occasions that Flynn comes back for a while) interact with each other and the villains. Stone finding historical clues, Jones circumventing security systems, Cassandra doing some crazy math in her head, and Baird keeping them all in line. There's a lot of character growth over the seasons and more than a few surprises along the way. Not too crazy with the surprises, though...it's a family show, after all, and nothing too terribly upsetting happens.
I had a lot of fun watching The Librarians, though I'm glad it ended when it did. By the time the finale rolled around, I felt the characters had changed enough that they were having trouble maintaining the team dynamic that made it so entertaining. The show wrapped up before that key component could degenerate too far, and in my opinion that worked out very well.
