What do you like best?
Pantheon's "opinionated" platform is well thought out, and the setup saves tons of development time compared to more basic hosting plans. Pantheon may as well be the gold standard for how to host -- how to keep sites running well, how to avoid common development problems, and how to develop more quickly because you're not fiddling around with creating a custom setup that keeps things running well.
What do you dislike?
The Pantheon Dashboard, once excellent, is now a barely functional mess. They said that a new version is coming soon, but I wish it hadn't broken so badly in the meantime. The problems don't stop me from getting things done, but they make certain tasks more confusing and a little slower than they need to be. I hope that they fix all the dashboard problems soon.
Recommendations to others considering the product:
Pantheon just works. See whether their plans are in your ballpark budget, and if they are, see if you can partner with a Pantheon-approved agency to get preferred pricing. You'll probably save money on the development end of things because your developer(s) will spend less time dealing with hosting-related issues like they would on many other hosting platforms.
What problems are you solving with the product? What benefits have you realized?
We don't have to worry about sites going down due to cowboy coding, poorly-coded plugins, unexpected updates, or sudden spikes in traffic. We've experienced a number of avoidable disasters before we moved to Pantheon, and now things run well.