Eh, it gets the basic monospace-preserve-whitespace stuff right. It's good enough for me 99% of the time, and if it isn't, I normally want to be debugging somewhere other than a chat app.
What Slack does very well is giving a good enough experience for programmers to do shop-talk, and ample opportunities for chatbot yak-shaves, it's also perfectly well usable for non technical people. I work for $ENORMOCORP, and after a couple of brave pioneers among the tech people, now everyone's falling over themselves to get their teams a Slack instance. (As others have said here, the ability to have multiple instances is pretty fundamental to this dynamic - we have team-specific Slacks, product specific, project specific and what have you. I certainly would not want some more 'old fashioned' colleagues seeing that NSFW bollocks people keep wiring into the tech specific chatbots...)
This strikes me as a pattern with some potential for growth - tech people driving adoption among non-technical colleagues. I reserve judgement on whether that's $3bn worth of potential.
If you gist the snippet beforehand, the experience is almost identical. This is obviously an extra step, but then your snippet isn't 100% tied to Slack. I have an app installed that gists my clipboard with a keyboard shortcut, and copies the link to my clipboard, so it's just an extra key chord.
