I've successfully moved my whole family, most of my friends, all my projects I work on and my "company" from either Skype or WhatsApp to Telegram. I counted all the people who started using Telegram because of me like 2-3 months ago and they were >200 - I think Pavel Durov should personally thank me for that Just kidding. But I really Telegram and can recommend it to anyone:
Great Device Support: (Windows, Linux, OS X, Android, iOS, Web and even BlackBerry)
Open Source: (Most of it, I think the Servers aren't but everything else yes. Clients, Protocol etc.)
And something I discovered myself which isn't even "advertised" by Telegram: Extreme high file sharing speed. I can send files with the size of ~500 MB and my "partner" will receive it after <2-4 Minutes (Me uploading and him downloading included!). I have absolutely no clue how they were able to achieve these transfer rates, and you got to have internet capable of providing these speed rates - but for someone coming from Skype, where even normal .psd files took years to transfer this is absolutely stunning.
Also awesome contribution @exodo! Really appreciate it.