As a software engineer here, I get to work on a huge variety of things, from processing 350,000 Tweets/minute via the Twitter Firehose using Cassandra, Scala, Redis and so on to our Rails app to JS visualizations. And we're moving and building fast, so we have learned to scale to millions of requests per minute. Yes, I get to work on things I care about, which is awesome.
The uppers put a lot of trust in you to get your work done and to do it well. You also have to proactive in choosing the problems you want to tackle. So if you're the person that prefers someone to dole out tickets for you to work on, this company may not be the right fit.
The culture is wayyyy fun. Vacation time is really flexible, and we go out as a company every month to do something fun. We've done Franklins BBQ, golf, swimming, volleyball, ping-pong, bowling, going out to bars, and others I've forgotten.