If you are a fan of Marvel movies and especially the Ant-Man series, of course, this is a must see. If not you can easily avoid it.
The best that can be said about "Quantumania" is it introduces the newest great Marvel villain, Kang the Conqueror, a time traveling supervillain who is a core Marvel character. In the film, Kang is played by Jonathan Majors and he is terrific.
Aside from a silly intro scene, the entire film take place in the sub-atomic world of the Quantum realm which is visually overwhelming and looks like a rejected idea from the Star Wars production team. Not only do many parts of the sets look familiar but some costumes and various creatures also seem very much Star Wars castoffs.
Returning to the series of course are Paul Rudd as Scott Lang aka Ant-Man, Evangeline Lily as Hope Van Dyne aka Wasp, Michael Douglas as Hank Pym, Michelle Pfeiffer as Janet Van Dyne, and Corey Stoll as Darren, now appearing as M.O.D.O.K. Scott's Daughter Cassie, now a teenager (with her own Pym Particle suit) is played by Kathryn Newton. Bill Murray also appears in a pointless and distracting cameo.
The film is overblown with special effects, noise and much running around with characters shooting and fighting each other. There is no real plot to speak of except for Kang and the others looking for a way out of the Quantum realm. To counter balance the chaos of the film, the writers attempt to add a human element, father and daughter bonding. "I love you peanut" just makes you want to cringe.
There are two extra scenes, both significant for a change. One mid-credits explaining more about Kang and one post credits, a complete and welcome surprise to any Marvel fan.
This film exists only as an introduction to Kang and the bridge to the next Marvel movie/TV show. If not already obvious, the filmmakers remind you at the end, "Kang will return".