What's impressive about it? Simple lists, a chat view, some text filter. Are you kidding me? Plus, just try "command-+" 2-3 times to enlarge the text, and see everything break down terribly.
But if you insist on web apps with more impressive UX work, anything in the list is better done: Twitter, Basecamp, Wunderlist, ..., hack even Gmail.
Sorry, but looking at it really doesn't do it justice. There are a lot of integration features that only appear when you need them. The UX is great because it's uncluttered and still powerful.
As an active Slack user who started testing out Inbox--there is zero comparison. Inbox has some slick items for sure, but is nowhere near as intuitive as Slack.
Polish? It looks and feels like a ho-hum single page app (which it is).