Pros
CloudFlare is a small company (100 engineers, another 150 in other roles) which has a big and growing impact on the Internet -- for the good. Rather than locking website operators into a single "stack" from Amazon, Microsoft, Google, or other hosting providers, CloudFlare lets websites choose how to host their site and then get top-quality network edge performance and security between their users and server infrastructure, for a price ranging from free (for millions of sites with less-demanding requirements) to about 10% of what incumbents charge for Enterprise customers. The biggest reason to work at CloudFlare is to be a part of making the Internet a better place. In contrast to a lot of other companies, CloudFlare has an extremely permissive terms of service, valuing free speech over everything else. While there are a lot of sites on CloudFlare which people find objectionable (for varying reasons), on balance, having open access at the network layer makes the Internet a better place. There are some big engineering problems to solve -- handling a huge volume of web requests means every other support system is also huge -- so it's a great place to get to work on hard challenges, the latest technology, and with some really smart people. CloudFlare has both a high media profile and a large userbase, so if you tell someone you work at CloudFlare, you're likely to get a positive response.
Cons
The biggest problem with CloudFlare is a legacy of flat organizational structure. For a long time, CloudFlare clung to a "flat" organization. This was great back when the company had 25 or 50 employees, but broke down at 100-150. CloudFlare now has a top-tier VP Engineering and some solid engineering managers, but is still implementing the kind of structure a company doing as much as CloudFlare does needs to have. For a long time, CloudFlare had below-market office environment (a cramped, poorly maintained space), but after 18 months of delays, we finally moved into an amazing dedicated building a block away. People visit and compare it favorably to any company in tech. If you want to be in San Francisco, the location is hard to beat -- a block from Giants Stadium and 2 blocks from Caltrain. CloudFlare cash compensation is essentially at-market. Equity compensation is less than market, but the company is also on a solid financial footing and good growth trajectory (because people actually need, value, and pay for the product), so it's hard to directly compare to more "bubble-based" valuations at other companies. Benefits are decent (free snacks, but only one free meal/week). Good health insurance, no other perks.