Diamond Peak is a very small, very friendly ski area. I skied there all the time as a little kid, and more recently as well. It is great for views, pricing, and beginners to easy intermediates. Everyone else, find another resort.
It is true that Diamond Peak has decent tree skiing, but it doesn't compare to nearby Northstar, Heavenly, and Homewood. Diamond Peak is very family friendly, but be aware that some lifts don't have safety bars.
The ski school at Diamond Peak is good, but not great. Back when I attended a few years ago, the instructors had me ride on a lift by myself with no bar! This was very intimidating for me, as it was one of my first times skiing.
Good value though!
See our video here

We visited Diamond Peak Ski Resort on a peak weekend, so as to be expected it was very busy in the hub of the ski area. There were kids everywhere and everyone was having a blast! We visited in a low snow season and there wasn’t much snow, but all the families and beginners there did not care. The base day lodge and beginners runs were right out in the sun, which wasn’t great for the snow quality, but it seemed to add to the enjoyment of the snow-ploughers.

This resort is great for beginners and low to mid intermediates. Most of the blue runs are pretty easy and some of the black piste runs are not too steep. The tree runs are the real pleasure for advanced riders, although it can be hard to find lines that go the fall-line for a fair way.

This is a community owned resort that has a matching friendly community vibe. It’s great for families and beginners looking for an affordable, no fuss snow experience. And if staying at the fabulous Hyatt hotel, the convenience of getting one of the shuttle buses to the resort makes it all too easy (particularly if it’s a peak weekend when they run out of parking)!

Also see the overview of Diamond Peak for our thoughts on the pros and cons of this resort and the USA ski resorts ratings to see how it compares to other US resorts.
See our video here