I don't think all movies should appeal to all audiences (you cannot be everything to everybody), and my guess is this film is like an "acid test" for creative people. If you are not of the creative-artistic type, the movie will probably say nothing to you. However, if you do belong to that group, you'll relate to a lot of what happens to the characters in the movie, and won't get enough of it; at least that's what happened to me.
Personally, some of the situations and questions discussed in the movie that I've faced myself as a member of the aforementioned group are: * Should I water down my production, in order to end the struggle of paying the rent of every month? * When you're in your 20s, there's something romantic about living like a band of gypsies, but as time goes by and everything takes more effort... where do you draw the line? * Seeing plagiarized something you never cared to protect in the first place, because you thought there was an understanding * Chickening out in the last moment from some "big opportunity" only because your gut feeling tells you that something's not right * Inspiration (the masterpiece that will come whenever) vs consistency (the product that, good or bad, you have to put at the door every week, following certain parameters, and nobody cares if your dad died yesterday) * That idea you have in your head and never get around to putting in paper * That work you've put in paper but never get around to submitting to other people's critique (because it's your baby) * Soul-drying day job vs still being financed by your parents at an embarrassing age The movie, like any good movie, does not provide a single answer to these questions, but each character's individual answer. Someone said in other comment that there is no character development, and I disagree; definitely all the characters end up in a different place to where they started, with lessons learned about the world, the others, and themselves.
There is a small scene that resonates a lot with me; when the protagonists meet Ben Stiller and they are kind of starstruck and overwhelming him with questions, Bill congratulates him for a Herman Monster sketch he wrote, and Stiller mumbles "that was a long time ago...". Here is the ambivalence: saying that something happened "a long time ago" is a bad thing in entertainment (it means you're yesterday's news), but a good thing in art (it means you've created a piece that people can revisit time after time through the years with affection). That tension is present in the whole film: are we just the court's jester, or is there something more important in stake here? The writing of the script is very subtle, it keeps growing on you after watching it (again, if this is your kind of film). Even the characters that appear only once or twice have a lot to them, amazing tips of the iceberg.
The choice of improvisation theater as the artistic discipline is also very wise; in all forms of art you are to some degree "putting yourself out there", but in improvisation that's perhaps more literal than in any other genre. You may have trained a lot but, when the moment comes, the piece of art is yourself, no safety net there...
The actors are all great, good casting and superb interpretations. Keegan-Michael Key is great in his role as, together with Stiller, the "celebrity-who-fights-to-stay-human". His love story with Samantha (Gillian Jacobs) is beautiful and multi-layered, there is even a short bed scene that is treated, for a change, with humor and simple kindness, so good to see after the usual "wrestling" that seems to be mandatory for those scenes in blockbuster films... The beautiful Gillian Jacobs can communicate a lot of emotion without using words, and she also has a knack for comedy... Mike Birbiglia is great as the slightly disenchanted leader of the group, who lives in kind of a "time wrap", a windowless room where he seduces his twenty something students, waiting for the next thing to happen in his career... The enigmatic Kate Micucci is perfect in her character of Allison, the delicate and insecure girl who draws her beautiful inner world, and always can use a push from her friends.
I also like the character of Lindsay (Tami Sagher), perhaps the most close to "conventional" among the troupe, with the blessing of wealthy parents that support her economically, which is also a curse as it increases the pressure to "make it". And Chris Gethard is great as Bill, a reserved guy who stays true to his principles and loyal to his friends no matter what, and in whom the balance he has achieved with his dad takes a very different shape.
Like I said, perhaps it's not a movie for everybody. It's an intimate kind of movie. I feel all those characters as my friends, as someone I could hang out with, but I understand that for other people, the movie can feel petty or random, like "why should they care". To each their own. :)