Pros
I've never before had a job where I voluntarily go into work on weekends... but I do that at FeatureX. It's not like anybody asks me to go in on a Saturday either, but I truly just enjoy being at my office. It's a beautiful space that really allows me to focus and get things done. It's an incredibly comfortable environment, so when I have some stuff I'd like to get ahead on, I truck up to the fifteenth floor and indulge in views of the Charles River while I send emails. And during the week, it's even better. I love being around my coworkers - they're incredibly smart, stimulating, and easy-going people to be around. We enjoy having lunch together, going on company trips, and have even been known to share a beer or two overseas at various AI conferences. Again, have never had a job like it. It's a great place to work.
Cons
There aren't cons. There are challenges with being in early stages, e.g. it's still a small company, no product yet. But those don't come across as cons here. They genuinely feel like opportunity to develop new technology, build a strong research team, and get hands-on experience of how to build a business.