Pros
This is the first place I've been at where everyone I work with is smart, capable, and a genuinely nice person. Other companies have smart people, but also have some bad apples. The work culture is very healthy with a lot of mutual respect. My questions about the business were never dismissed as unimportant or irrelevant, and I'm learning a lot. Our remote-friendly culture gives us access to great engineers that other companies wouldn't be able to hire. Our codebase is clean and pretty easy to get around. Also, because of the culture, everyone is eager to help and answer questions. It helps that our CEO is an engineer, and actually know what it takes to write good code.
Cons
Heap may not be the best place to start your career. We're still small enough that we expect everyone to come in with a solid understanding of the stack of technologies used on the web. The company generally treats you like an adult, so you're expected to know what you'd like to work on and be capable to pick your own project from the list of company priorities. Also, if you're into startups with strong partying / drinking cultures, this is not the place for you. We have fun events and onsites, but the employees are level-headed adults, so you won't get to enjoy the shitshow that generally ensues at other companies' events.