I've used it at eBay, it runs all the production servers at my current company, and anything I launch on AWS nowadays is on it. There is a very very distinct separation between Ubuntu for desktops, and Ubuntu Server. Even at the most core level, desktop Ubuntu's kernel process scheduler favors short-running userland applications that need to support many different simultaneous processes and threads, while Ubuntu Server is fine-tuned for long-running processes that benefit from minimal thread switching. Debian's kernel strikes more of a balance between the two, which, right from the core, gives Ubuntu Server an advantage.
It also comes with administration packages that are pre-configured for Ubuntu's setup, not to mention Ubuntu Server's service management system that gives you a better grip on daemon processes than the independent scripts that Debian relies on. It's also managed and maintained by a company with a vested interest in the platform and control over the codebase, which has proven beneficial to server OSes in the past (See RHEL) because contributions are better curated and release/update schedules are better controlled.
Not to say Debian is a poor choice for a server OS -- until relatively recently, Debian has been my go-to. Ubuntu Server is an extremely sane choice now, though, and shouldn't be dismissed just because of its name or the fact that it gained popularity on the desktop.