MailChimp is very good. It can be clunky sometimes when you use the templates, but overall, we’ve been very happy. They’re very inexpensive, or even free, for many companies for a long time, which is a huge advantage of using them.
We’ve been using Algentis, but they were acquired, and are now getting out of HR and payroll. For now, we’re doing PTO tracking through Google Docs until we switch to a new provider.
Buffer is okay, but it’s definitely not a perfect system. Sometimes, the timing gets screwed up, and other times, you can load a post in and it takes longer to load than it should. If you’re doing a lot of social media, but don’t care if a certain post goes out at exactly the right time you’re expecting, Buffer is fine. But if you do care, it can be a major problem.