I use Slack every day at work, and this is a really useful feature. I’m terrible at remembering all of the text formatting markup.
There is one thing which I’d change however. Once you have been typing for a while the tips disappear, and there is no way of making them show again, other than deleting your draft message and starting again. This is particularly frustrating as I tend to need the formatting hints most when writing longer messages!
Link - Slack: when you start typing in the chat, tips appear to format text