Really, what more about this album has been said that hasn't been said already? Rust in Piece is Megadeth's Magnum Opus, along with Peace Sells... and for good reason. This is the point where the band took their complex progression and multi-sectional thrash to its logical extreme, before regressing into more standard metal with Countdown to Extinction. Through the creative sections along with masterful riffs, this shows that Megadeth were far from just screwing around with music.
There are plenty of arguments about Megadeth vs Metallica, and one reason people see Metallica as better are their long-form progressive metal tracks. While the tracklist shows that the song lengths for Megadeth aren't excessively long, Megadeth preferred to throw out the repetition on albums like this and bring tons of interesting sections into a well-constructed, concise format. One of the best examples of this is one of the deeper cuts on the album, "Five Magics", which starts off with some repetitive bass and guitar interplay, before blasting off into multiple high-energy sections, ultimately climaxing into a gigantic buildup. In less than six minutes they say more than, say, the eight minute opuses "Battery" and "Master of Puppets"
They do not suffer from sporadic changing syndrome that causes some to groan when listening to the more instrumentally challenging bands. All the riffs are solid, where Mustaine and Friedman just crank out great riff after great riff. The opening shouting of the guitar kicks it off in "Holy Wars... The Punishment Due" and doesn't stuff. Most of them are energetic and thrashy, peaking out speedwise at the rockabilly-esque "Poison Was the Cure". Other times they are slow, like the latter section of the opening track. No matter what, though, they all pack a punch, with maybe the sole exception of "Lucretia" which while opening with a great melody on guitar delves into some sections that are slightly more standard in metal.
Again, Mustaine and Friedman must be mentioned. Friedman is many a Megadeth fan's favorite guitarist, and the solos on this album back it up quite well. It's difficult trying to tell what brings more to the table, the wild, endless soloing that goes on and on in "Hangar 18", or the emotional and shreddy blast that is "Tornado of Souls". The former is quite entertaining, with Mustaine and Friedman going back and forth against each other in a chaotic maelstrom of fury, getting faster and faster and building in increasing rage and technique. The latter, however, might be Friedman's masterpiece of a solo, containing thousands of exactly the right notes needed, giving a wonderful emotional and energetic touch needed in such a powerful song.
At the end of the day, you should know what you're getting into with "Rust in Peace". Megadeth is at the top of the game, and while the thrash isn't quite as extreme or dark as other bands in the genre, this still stands as an important pillar in the world of metal. Plenty have stated its influence and importance, and the music behind it speaks for itself. So if you're reading this and haven't gotten it already, there's really no excuse. Go out and get it.