I agree this was a major problem, I have been using G+ off and on since the very beginning. It was annoying that I could not select an available username instead of the one Google thought I should use, and I wasn't happy that I couldn't use /c/myusername even though I knew it was available. No consistency whatsoever in the UI/UX. It should have been a cinch to sign in on the same username across all Google services.

You're also correct that this is something you'd think a company like Google would be able to execute well on. Well, you guessed wrong...