Mr Pibb/Pibb Xtra is in the same category as Dr Pepper, IMHO and Coke had originally named it Peppo to target the Dr Pepper market; incidentally Pepsi doesn't have anything in the "pepper-drink" category). I don't have a lot lot of familiarity with Pibb, because this category isn't something I desire very often, and Pibb availability is generally low; but next time I see it, if it's not one of the hideous Coke Freestyle machines where the flavors come out of inkjet cartridges, I'll give it a try and update my references. There's some similarity in initial flavor to cherry Coke, but you know, there's cherry Dr Pepper out there too, and that's distinguishable from regular Dr Pepper.

I must admit I haven't had birch beer; but I'll try it when I see it too. :)

Pibb used to be cherry-forward, smoother-drinking Dr Pepper (nb I preferred Dr Pepper).

Pibb Xtra is mostly just SUGAR punching you in the taste buds.