Keystone is staying open until 4/20.
Snow coverage will be good, surface conditions will vary based on the weather/sun/temperatures during your visit.
Like all spring skiing, follow the sun to get the right consistency of snow.
You could have good corn, you could be there during a storm cycle and have mid-winter snow conditions.
If it is a really warm day (50's+) I would tend to avoid Keystone since it will get too soft/slushy faster than places like A-Basin or Breck's upper mountain.
Crowds will be non-existant, even on weekends in April. The Front Range skiers and tourists, except for the die-hards are done for the season. Plus, those that do ski tend to gravitate more toward Vail, Breck, and A-Basin. I'm actually surprised Keystone it staying open until 4/20 this year, I think this is 1-2 weeks later than most seasons (probably due to a late Easter).
They typically close the Mountain House base area around the end of March, with only the River Run lifts open on the Front-side after that time.
I've had some great spring days at Keystone, other times I've avoided it, but it also depends what I've been in the mood to ski.