Cross platform like almost no other messaging app. Super fast and reliable. Not tied to any ad-hungry company account. By far the best.
No way to switch between a work and personal account on the same device, or flag conversation/contacts as such. Also no SMS mode (GroupMe).
I've tinkered with Telegram for years, but really dove in over a year ago when I grew tired of Skype for Business failing, Slack feeling too cumbersome, Google not having their act together with...well pretty much anything to do with messaging, and FB/WhatsApp being too ominous/stigmatized.
I have used Telegram on iOS, Android, Chrome (browser & OS), and Windows. It just works. If I could get everyone I chat with to use it instead of one of all the other ones, that would be a refreshing change.
The one app that has an advantage over it is GroupMe because of SMS mode. A person can opt into receiving and sending messages via SMS only. They lose a lot of functionality, but it's still possible and works well.