I started out mixing flavors at 20% and was not pleased with the results. I wouldn't say it was flavorless . .but the flavors didn't taste like they should. I have gone down to max 10% total flavoring (I use fruit, mint and bakery flavors) and my juices are much better tasting.
I realized that e juice is like wine in that the flavors should invite you in . .not punch you in the nose.