History
Picture 2001. The 9/11 attacks had just happened, the world was entering an era of mass surveillance, and a programmer named James Yonan sat down to write code. He didn't like how existing solutions like PPTP worked (which would be cracked even in our grandfathers' lifetime), so he decided to build something open and flexible.
That's how OpenVPN was born — the grandfather of modern protocols. It wasn't tied to a specific OS, used the powerful OpenSSL library, and most importantly, allowed you to configure anything. Since then it has passed through thousands of audits, survived attacks from intelligence agencies, and became the workhorse that still carries half of the corporate VPN networks in the world.
Advantages
Why is it still alive? First, versatility. OpenVPN can work over both UDP (for speed) and TCP (for reliability and obfuscation). Second, encryption flexibility. You can decide yourself whether to use AES-256, ChaCha20, or something exotic. And most importantly — stability. It's such old and well-studied code that bugs are rarer than typos in a math textbook.
Disadvantages
But there's another side of the coin. OpenVPN is slow and heavy. Its codebase is a monster of hundreds of thousands of lines. Because it runs in userspace rather than in the kernel, it constantly copies data back and forth. On weak routers you'll drop speed several-fold. Plus, it's easy to identify. DPI systems know OpenVPN traffic signatures like the back of their hand.
Summary
OpenVPN is a Volvo 740. It's heavy, not very fast, guzzles gas, and doesn't shine in design. But if you get into an accident (or a hacker attack), it will most likely save your life. For corporate security and those in no hurry, nothing better has been invented yet.