Pros
The work you do affects virtually every important company that has a website, every single day. The company's foundations -- financially and technically -- have been laid down very well. There's little doubt that the company will continue being at the forefront of the CDN industry for years. The benefits are terrific. The company skimps on everyday perks (e.g., providing lunch during meetings), but in exchange they focus on providing solid bonuses at the end of the year and providing solid base salary. The company was the dot-com bubble's poster child (check out their stock history), which came crashing down very quickly, and they had to lay people off a decade or so ago. Consequently, they've been very conservative in the years since. I'd be surprised if they'd ever lay anyone off again, short of a disaster. In an economy with companies that don't hesitate to dump their employees, that's worth a lot.
Cons
It's kind of slow and stodgy. Management has recently been making a lot of noise about 'innovation,' less because they seem to know what that means in this company's context and more because that's a word that everyone uses. The real innovators in the industry -- the Googles, the Facebooks -- are leaving Akamai in the dust. Akamai doesn't even really register on the map for people coming out of college with CS degrees. Akamai should be a company that keeps people in Cambridge, but it's not. I doubt many of the top candidates from MIT and Stanford and CMU have ever decided to come to Cambridge because Akamai offered them a job.