All my previous dogs have been mixed breeds. I just adopted my first purebred. I'm not concerned about pedigree as much as I am temperment.
Was it an impulse decision? Depends on which dog!

When I adopted Libby, an Australian Cattle Dog mix, it was semi-impulse. I was looking for a dog, went to the pound, saw her looking quite terrified in a corner of a very large kennel and decided to adopt. I adopted the next one completely on impulse when I went to a trainer seminar at a PetCo and saw him with one of those rescue groups. Me and another couple raced to see who got approved for him first.
This week, I adopted a purebred cocker spaniel from a local rescue group. This time, I did research. I considered breeds with an eye on what would work for what I wanted (athletic dog) and what would suit my family. Cocker spaniels seemed the best fit. I found several on Petfinder.com and did some meet & greets and decided on Tanner.
Would I choose another breed? Tanner is wonderful and exactly what fits in my family dynamics at the moment. Under different circumstances, I'd love a rottie and/or a collie...or another cattle dog.
I think knowing the breed is nice but ultimately it's more about energy and temperment than breed.