I'm such a sucker for Sherlock Holmes' adaptations! And this is a great one. Martin Freeman is so fucking awesome. He's a brilliant Dr. Watson. Honestly, I could see him playing a tree that I'd enjoy. Benedict is amazing as well. This is a great first episode.
What I love most is the kinetic energy brought to this adaptation. Cumberbatch holds a magnetic presence on screen, and I never tired of his character. The duo of Watson and Holmes is one of the greatest in all fiction, and Moffatt and the two leads do a great job of bringing the dynamic and chemistry to life. What worked the best was how believable the characters were. They felt fresh and real, not caricatures.
As a huge fan of Sherlock Holmes, I was very satisfied. Clever, fun, and Cumberbatch and Freeman are both great. An ideal example of great adaptation. It captures the essence, modifies what it must, and makes additional changes that allow it to feel fresh and original. But most importantly, it stands alone - you don't have to be familiar with Doyle's work in order to appreciate it.