Our group just recently fought and killed a yound adult CR 13 red dragon. If we'd not prepared for it and know it was there, we'd be dead in a heartbeat. We got a few spells through (mostly no-save ones) but it was the fighters and the rogue/cleric that took it down. Still, they hurt badly - with low 20s ACs that dragon was power attacking like mad and each blow hurt like heck. A lot of spells fizzled on the SR as well. Overall, though I think they're fine - they're meant to be resistant to most things. While fighters might have an easier time, they still drop really easy as well given a dragon's insane number of attacks and Power Attack.
Pinotage
Pinotage