Many people might disagree with me,but Evelynn....
Has one of the more healthier assassination patterns.
Her overall gameplay is toxic as fck.Nobody likes to play against perma-stealth,but her assassination better is one of the better ones.
She actually has to sneak up to you,she lack reliable CC,doesn't have any of those annoying dashes/blinks/mini teleports like other champion.
Her early is weak,in the sense she can't really kill many champions in 1vs1 scenario.She goes either to a lane to 2vs1(3vs2) someone or just sneak up to low HP targets.This changes late game,where after doing well she can actually burst squishier targets,but still leaves at least a small window of opportunity for the opponent to react.
Like every champion she has her own issues,but for an assassin she's pretty much balanced.
but with the new jungle and new 
I usually just go AP assassin now.