I've actually gotten really into Streak! I think it's a excellent and intuitive in-email option. I live in my inbox, so it's right there and just keeps me on top of everything. I've been very happy with it.
We tried Slack for a while. To be honest, HipChat is an ongoing process for us. We mostly use email. When we tried to switch over to Slack, it just didn't really catch on. We use HipChat more as a repository for interesting stuff than as a way to communicate internally.
We don’t use MailChimp much, but we chose it because we're creatures of habit. We'd all used it before, and it seemed like the simplest and easiest UI. I've gotten used to it over the years. It just seems like the best.
We've had frustrations with Box. They have UX issues, like files not syncing properly or not updating. We picked it because it has a way of providing a secure upload link for customers. But they have problems, and we’ll probably be moving away from it soon.