I first read the Dark Phoenix Saga (as individual comics) during college in the early 90’s. Given the release of the Dark Phoenix movie, I thought I’d re-read it to see any similarities. I’m not going to spoil either the comic or the movie, but I will say that similarities between the two are loose. Very loose. That said, the comic omnibus is fantastic. It covers Uncanny XMen issues 129-137 wherein Jean Grey is struggling with the Dark Phoenix, a cosmic entity that coinhabits her body and slowly gains control over her.
This is a classic XMen story arc and it occurs at a very key point in the history of the Xmen with many long-lasting effects going forward. Events from this arc essentially set in motion enemies for the XMen that lead to Days of Future Past, the Sentinels, the political adversaries that lead to the mutant registration act, Genosha, and essentially sets the broad course of the XMen for at least the next 20 years in print.
The artwork by John Byrne and Terry Austin is absolutely fantastic. It was IMHO the best artwork in comics during that era. The other thing to keep in mind is this is what lead into the explosion of mutant comics at Marvel. The XMen was changing. There’s something of a reunion of the original teammates with the new members during Dark Phoenix. Not long after this run, mutants would go from 1 comic (Uncanny Xmen) to so many that it was difficult to keep up – X Factor, New Mutants, Wolverine, Excalibur, Longshot, XMen, and I’m sure there are tons of others.