It depends on what I'm baking, what it's for, & how much time I have!
I always use a mix for brownies though.
However, w/ cakes & cookies, it depends...
For instance, I make cakes like Hummingbird Cake or Carrot Cake from scratch. And I'll make quick breads from scratch like banana or zucchini bread.
I made a homemade Strawberry Yogurt Cake at Easter that was really good. I also have a recipe for "Better than Sex" cake that's a "from scratch" recipe that's yummy... but, honestly, not as yummy as it claims.
At Christmas time, most of the cookies we make are from scratch except the sugar cookies for which I use the Betty Crocker mix. I'll also make a couple of cake mix-cookie variations as well.
I'll also use a cake mix for various cakes & different recipes. There's a Key Lime cake I make that uses a lemon cake mix.
For birthdays, we usually buy the cake from a bakery. For my kids, part of what makes their birthdays "special" is an elaborately decorated/iced cake. And their momma can't do elaborately decorated cakes!
I tried twice - once for DD's mermaid birthday & once for older DS's pirate birthday... and then I learned my lesson.
I used to always use store-bought frosting & then I figured out how easy home-made frostings & icings are, so, now, I usually make my own frosting - same thing w/ whipped cream. Easy & so much better tasting & better for us!