Super underrated. Went there with some friends for the 4th of July weekend and I was completely blown away by some of the views. I personally feel it is on par with Yosemite in terms of pure beauty, and it can be experienced with a much smaller crowd than more well known national parks.
When we visited, my friends and I backpacked for an entire day, hiking a beautiful trail from the visitor's center up to the famous Bumpass Hell area. Lassen Peak itself is gorgeous too, but unfortunately we ran out of time before getting to hike it. I actually have plans to return in a few weeks just to finish that hike.
We didn't even visit the eastern side of the park, but there are countless things to do there too.
Wonderful park. The drive there is beautiful, and it is perfect for backpacking.