It's one of those too-obvious things.
I had been doing some development with 4DOS batch files and caught myself typing echo $? so often I wondered why I couldn't put it in the prompt. Of course, I actually could, but it took me a few minutes to realize that! When I later picked up Linux and bash, I'd gotten used to it in my 4DOS prompts, so I put it in my bash prompts. Glad you like it.
Wade.
"Ah. One of the difficult questions."