The least used instrument in the cockpit by way too many pilots. I finished up my instrument ticket with IPC and we flew so much partial panel IFR that I developed a real A game at it. And then proceeded to not practice it nearly often enough to count on it. I started swapping approach practice rides with a good friend who was a CFII and he'd always drill me on partial panel. I lost my HSI once on approach in IMC. It was only the constant training I'd done flying no-gyro approaches at Andrews AFB that saved me that day. I decided that when I wasn't flying enough IMC (or practicing it enough to have that A game) that I stopped launching into hard IMC. I eventually got ride of the Columbia 400 and was having more fun flying the Pitts anyway.That one simple instrument can save your butt.
Speaking of the Pitts, Bumblebee, did you know that you can use the ball when it's inverted? There's an air bubble in it that functions just like the ball when you're flying inverted