It's a diner, nothing more. But in a town where I could eat breakfast for $20 at the Dry Creek Inn, the Singletree offers a nice contrast to the plethora of cools spots and latte being walked around the square.
I got 2 eggs over easy, country potatoes, and an english muffin all cooked fine. My eggs were over-easy and not burnt around the egg white. The potatoes weren't particularly crispy, but they were seasoned heavily and the muffin wasn't burnt. Not much to complain about and everything was brought to the table quickly after ordering.
So if you're in Healdsburg, perhaps staying on the square and you don't want a a vegan oatmeal muffin and a latte, walk down toward the train tracks on the West side. The Singletree isn't fancy, but it does have breakfast for under $10.