Pepsi Blue is good. It doesn't replace Mountain Dew, but it's good.
Vanilla Coke definitely replaces any other Coke-brand cola I would ever buy, but it still tastes too much like Coke to be any good. Pepsi proper is so much better.
And I never liked the lemon colas much. I accidentally squeezed a lemon into my Pepsi glass once (instead of the water glass sitting right next to it which I meant to put the lemon into), and it didn't taste all that bad, but the packaged lemon colas just don't do it for me.
---Jamin