Comic book purists tend to forget a few things - that actors are capable of doing more than one thing, and that movies as a medium are inherently different from comic books, and should therefore not be the exact same thing as comics. Think of all the unexpected actors who turned out to be great in their roles - Anne Hathaway as Catwoman, Michael Clarke Duncan as The Kingpin, and Robert Downey Jr. as Iron Man (for real - RDJ was some recovering junkie before he became The Coolest Guy In Hollywood after Iron Man).
Also, Heath Ledger as The Joker. Before that, Ledger was sorta a pretty-boy whose last big movie was Brokeback Mountain - hence a lot of gay panic jokes and talk about how he was too much of a handsome dud to effectively play the Joker. AND WE ALL KNOW HOW THAT TURNED OUT.
Still, looking back should give us all a pang of shame.
Basically, it's best to not judge too hastily. Give Batfleck a chance.