Your friend has no idea what they're talking about. Shadow plays somewhere in the middle of the spectrum between a straight nuker (Arcane mage) and a pure dotter (Aff Lock).. with only 2 dots and bomb spells like Mind blast, void bolt, void torrent, it will be a refreshing change of pace for you.
It's not without flaws though. As others have said, Aff locks are just overtuned right now and Spriests are more gear (and legendary) dependent... and require better internet, but that's another thing. As the saying goes, Spriest is a poor man's Affliction. Personally I play my Aff lock when I feel the need to stomp meters and parse while drooling on myself, and play shadow when I want to have more raw fun. Bonus: Shadow pvp is a blast!
In the end it's up to you, but maybe consider giving Shadow Priest a chance.