Stable conditions, low vis approaches and takeoffs, it's a great addition.
A worthless distracting POS in rough weather and turbulence, light
polluted fields and night and BS requirements to use it VFR. Great resource for
weak crewmembers with little real experience before flying a transport
jet.
Used an AA one into SJO Costa Rica at around 300/1. Following the
HUD guidance took me past the touchdown zone.
Once you learn to look through it rather than at it, the HUD provides a wealth of useful information in all flight regimes. It does not, however, fly the jet for you.