Well, I personally love Slack and I use it at work.
Outside of work, what most people I know use are the usual suspects - WhatsApp, Messenger, Telegram. These apps are great for what they do well, which is mainly one-to-one chat, though I don't deny that they all have group features as well, albeit pretty basic in my opinion.
Some issues with Slack are:
1) They're geared mainly towards enterprise users, reflected in their features as well as their pricing model. Though admittedly, they do a good job in making it fun too.
2) Slack interest groups aren't easily discoverable.
Discord is basically Slack for gamers and that's where they excel in functionality and features.
IRC is what I grew up with, and it did play a huge part in me building Wander. If I were to sum up what Wander is (for the technically inclined), it would be Slack meets IRC. A modern, mobile and simpler alternative to IRC channels with the fun and interactivity of Slack in chat.
