[myrion]

They don't though. Evolving gives you, in my experience, +100, perhaps +200 CP.

All my 10-50CP Pidgeys don't magically get useful by evolving. Though they do give nice XP.

[calsy]

Yes they do, my most powerful characters are all margikarp, psyduck, eevee, exeggcute evolutions. I have a 990 CP Pidgeot from pidgeys.

[xbmcuser]

Power them up before evolving them.

[mdrzn]

Evolving doubles the XP usually.