Two weeks ago I purchased one of each of these headlamps directly from Zebralight. They have both received high praise on various web forums, and it happened the manufacturer was having a sale ($49.50 with free shipping). At that price, it was almost in the territory of the Petzl Tikka XP2 lamps, so I jumped. They arrived quickly and I was impressed with the light output ( 200 lm for the H51 and 220 lm for the H31) and the build quality. These are both IPX7 waterproof (2 meters for 30 minutes). They came equipped with headbands, additional O-rings and a "glow in the dark" headband attachment that can be swapped out for the standard black one.
I liked them enough to go back to check on buying another for a gift, only to see them listed as discontinued models ( all variations - flood, neutral white, etc.). Price is $64. It appears Zebralight is focused on their H600 model - cost is $89 - although it was not available for purchase on their website at this time. This light puts out substantially more lumens (1,020) on high and takes a special 18650 size 3.6/3.7V li-ion rechargeable battery which appear to cost $4-$7 per battery plus a charger station.
They still offer the H502 ($64) which takes AA but is a flood design.
Not sure I would ever need 1,020 lumens and I know I am not ready to pay $96 (including battery) for a headlamp. Somewhat surprised that they would discontinue such a popular headlamp - happy I got one when I could.