I will try to leave opinion out of this until the end.
System of a Down has made 11 albums (6 of which are demo albums). As time went on, most would say they got more popular, but made less quality music. System of a Down has many different types of music (it's hard to believe that Lost in Hollywood and Sugar are the same band). System of a Down has evolved quite a bit from their first album. Before anyone insults them, listen to at least 2 songs from each of their albums. Before you compliment them, do the same. Don't say they're sellouts. They gained popularity, but what else would you expect from a band?
Now for opinion.
System is amazing, and has lots of talent. They've been my favorite band since I first heard them. I frequently listen to each and every one of their songs. System is one of the best bands ever. They don't like saying what their songs are about (they prefer to leave them for the listener to interpret), and try hard not to say. Some of their songs are about drugs (Prison Song), some about sex (Bounce), some about genocide (PLUCK), some about nothing (Vicinity of Obscenity). I recommend their unreleased works; they're much better.
System of a Down's albums (demos excluded)
System of a Down - 1998
Toxicity - 2001
Steal this Album! - 2002
Mezmerize - 2005
Hypnotize - 2005