Pros
If you're the kind of person that deeply cares about games and wants to really enjoy going to work every day, Riot is definitely worth considering. The perks are generally excellent, but it's the ethos and the other Rioters that really make it special. While sometimes daunting, it's awesome to feel dwarfed by your talented colleagues all across the company.
Cons
We grew really big, really fast. As a result, there's a ton of inefficiency and patchwork solutions that often make it difficult to get a particular project done. A lot of people (parts of the League of Legends community, some Rioters, etc.) don't realize or appreciate the path that was taken to get here, and they don't realize that many of the "bad decisions" that were made early on and have resulted in a difficult tech base or difficult processes were a result of a company in uncharted territory doing its damnedest to keep their head and service above water. It doesn't mean that the current pains aren't real, but it's important to keep some perspective. One specific con of working at Riot is that, mostly as a result of a very large, very dedicated playerbase, the amount of vitriol and anger (and misinformation) that can get thrown around by people on the internet towards you and your teammates can be pretty rough. If you have thin skin, it can be hard to dive into player conversations.