Pros
Great perks, mostly good engineers, some cool technologies to learn and work with. I joined Twitter several years ago, when there were a lot more opportunities than now, and was able to design and implement something new and cool enough, that made a significant impact.
Cons
I was a tech lead in my group and made many contributions to several projects. I got a second salary rise, the management once again told me how awesome my work is... and ten days later, I got laid off. After that, several engineers from the group wrote me privately that they are shocked by this decision, that my productivity and overall impact was exceptional, etc. Make your conclusions. I can confirm what others have already said: at some point the company started to bring in a lot of middle managers from outside who turned out to be not very competent at best and political at worst. As a result, I've seen, in particular, cases of groundless criticism of hard-working people and apparent favoritism towards some lazy ones. Engineers who did almost no work for months and were "in meetings" all the time, were nevertheless treated with respect, asked to write one useless design proposal after another, etc. Seeing all this, young and dynamic guys with a proven record of success would leave the group and eventually the company, while the management would happily hire twice more of new, less experienced and energetic people.