Avantages
PromptWorks has a wonderful culture of learning that I really appreciated and which leads to you working with other curious engineers. The small size is also nice because you can have a bigger impact. They have company lunch twice a week, one day where someone will generally present something, be it technical or not, and other where they give a sales update and give shout-outs to employees. This helps create a nice environment of knowledge sharing and a sense of involvement in the company. There's also room for employees to get involved in different initiatives as the company is at a size where the founders are looking to delegate.
Inconvénients
Though benefits are good, pay is below average. Since it's consulting, work can be hit or miss. You may be on a super interesting project for 6 months with a great client and then a terrible one. They also do a lot of pairing, which I'm not a huge fan of. Much like TDD, do it when it makes sense, not all the time. Also not a fan of no one having a permanent desk.