Yep, I do remember those tediously challenging music class assignments of having to learn & play an assigned tune in all 12 keys plus turnin' in papers with them fully transcribed and handwritten in 12 keys too. It's a relief not to have to go thru that now. Those were the days no longer. And because in addition to the most popular guitar sharp keys (E,A,D,G), I especially enjoy working with horn players, so over the years have developed a fondness for their favored flat keys (F, Bb, Eb and Ab) and which seem to more often than not, best suit my voice as well. In addition to 'C', having these 8 keys under your belt will serve you well both on and off the bandstand. Keep in mind that musicians (esp horn players) without a convenient automated transpose option (transpose button or guitar capo) can't easily transpose on the fly to a key like C# or F# and won't appreciate a singer or piano player coming on stage and calling out "Crazy" in C#. If I'm working with a pick up band, I've got enough latitude in my singing to call the tune in C or D instead, and believe me, the band will certainly appreciate that and calling you back to sing/play with them again more too.