First off, this is coming from a huge Christopher Nolan fan, so my opinions might be a little biased.
I think Oppenheimer is a good movie diving into the man himself and how his actions affect him and people around him. The acting by Murphy (as the rest of the cast) is amazing and we get some very strong scenes with great impact executed well. The movie does a decent job of keeping your attention at most times however in a 3 hour long movie at some points you might feel lost in that political lingo or find yourself questioning the importance of the scene.
The movie has a touch of a courtroom drama like JFK with heavy political driven story not everyone will enjoy. Those who do certainly will, however at times you find it difficult to find the impact of a particular scene. A lot of the movie focuses on whether or not Oppenheimer was a communist, and that speaks about how people keep changing to be on the right (subjective) side (Oppenheimer being hailed as a hero by the country but after thinking through what happened with the Japanese, they question his loyalty). Whether or not he is a communist occupies a larger part of the movie than it should have. Further the weight of characters is pulled down by the sheer amount of characters that are present, some of them getting more time than they really need, and it doesn't affect the audiences (but alas its not fiction, you need to have those characters)
The movie can get confusing at times when it jumps between timelines and requires your attention but if you're into it, its not that hard to catch but just as easier to lose your way. Performances from Downey, Damon, Pugh and Blunt aside from Murphy are very strong and their characters very much add flavour to the movie.
I believe the hype surrounding the movie, the way it was marketed and hence the way it seemed it would be, and the way people expect a Christopher Nolan movie to be just puts some wrong expectations in mind, but if you approach it with a fresh mind, thinking it's historical biopic, it will serve you much better.
It is not a story about the atomic bomb or its creation but rather a movie about the man himself and understanding him, and I believe the movie does a fair job at that.
Overall for me, it was a good movie with good engagement and really strong acting. However, got a little too political compared to what I expected, and moreover some scenes and characters seem insignificant and do not contribute to the story and seem like they're just there to be there. Might be in contention for some oscars as this is the stuff they look for. Not Nolan's best work (I hope that one's yet to come). A great and importantly unique biopic. Surely more than a 4/5 but again not a 5/5.
P.s. Cinematography is top notch, and the music is good.