I'd say a boost gauge is pretty much essential in a re-mapped car, even in a standard turbo'd car I'd say it's pretty useful.
Most 80's turbo'd cars had them as standard but they seem to have gone out of fashion as standard fit in modern cars.
First it helps spot any early signs of power loss due to a hose leak, secondly it can
help you see when you're spooling up ready for an overtaking manoeuvre (did I spell that right??) and avoid that turbo lag moment.