The spell says it ends if the steed takes damage, but it also says the steed takes a full minute to vanish "when the spell ends".  Does that mean you actually have 10 rounds of combat before the steed disappears if it takes damage?  Because that's actually a long time.

Yep, 10 rounds of zipping around the battlefield (or away, for a 1000 feet retreat).

Phantom Steed is a hidden gem of a spell. A Wizard can ritual cast it over and over, gradually transporting the party over great distances for no cost. Only issues are potential ambushes.

Really cool summoning spell that is also a ritual (much like the beloved 'Find Familiar' spell).  In my mind this is more of a conjuration spell but I understand the arguments why this counts as an illusion spell. Very strong spell IMO. It's a ritual, it requires ZERO concentration (so the whole party can have these steeds?), and it's beneficial for every class. I like this spell :)

Quote from chewy_the_unaware >>

Really cool summoning spell that is also a ritual (much like the beloved 'Find Familiar' spell).  In my mind this is more of a conjuration spell but I understand the arguments why this counts as an illusion spell. Very strong spell IMO. It's a ritual, it requires ZERO concentration (so the whole party can have these steeds?), and it's beneficial for every class. I like this spell :)

While there's nothing preventing you from casting it more than once, it might be tough for a single caster to give a whole party phantom steeds at the same time, because the spell takes 11 minutes to cast as a ritual, and it only lasts for one hour. 

Quote from wagnarokkr >>

While there's nothing preventing you from casting it more than once, it might be tough for a single caster to give a whole party phantom steeds at the same time, because the spell takes 11 minutes to cast as a ritual, and it only lasts for one hour. 

“For the duration, you >>>or<<< A creature you choose can ride the steed.”

“A creature” means only one, and you can choose yourself OR that one creature.

A Monk with a couple levels in Wizard could use Expeditious Retreat and a couple Tenser’s Floating Disks for roughly the same effect. The limitation is 500lbs per Disk and Expeditious Retreat only lasts 10 minutes. That should be enough for 3-4 light weight casters, 1-2 armored players, or up to 500 lbs of loot on one Disk.

Depending on your Monk’s build and Level, your speed could be 50-100 and Expeditious Retreat means you would move 50-100 for your Move, your Action, and your Bonus Action: 150-300 per turn. After 10 minutes: 15,000-30,000 feet or about 3-6 miles.

If there are 10 rounds in a minute, and it moves 100 feet/round, then it would move 1,000 feet in a minute and 60,000 feet in an hour: 11.36 miles/hour.

Are they using dice to calculate these numbers?

It clearly says that it fades slowly giving the rider time to dismount. It is not to let you ride it for another minute.