I found him pretty bland. He felt like a generic dark lord evil for the sake of evil baddie, which isn't terrible in itself, but unfortunately he didn't have the same presence other dark lord characters have had in other stories, such as Sauron or Palpatine.
As others have said, your interaction with him was very saturday morning cartoon 'I'll get you next time Gadget' type stuff.
His forces never really felt like a threat, as after Haven you foil his plans at every turn.
His big bad dragon also never felt like a threat. This is admittedly a bit of gameplay/story segregation, but I could just not be worried about a dragon after having already taken out a dozen more while just roaming around the world. Even the fact it was a darkspawn dragon didn't mean much as I'd also taken out so many darkspawn. Honestly, it was actually really annoying how everytime my character faced the dragon I got forced into a cutscene where my Inquisitor was falling over herself to get away. I just kept begging for the game to let me just fight the damn thing and get it over with, because I knew I could take it.
Corypheus himself sort of felt the same way. I was never really given a good sense of why I couldn't just run up and engage him in a fight anytime I saw him, because he never came off as that magically or physically powerful. Yeah if he dies he transfers into a darkspawn or warden and comes alive again, and that's a problem, but that's still no reason for me not to fight him, and it's certainly no reason for my Inquisitor and her companions to be forced to run away from him during a cut scene like he's a hundred foot tall unstoppable juggernaut.
Also from a visual point he just wasn't that interesting. He was just a more blinged up abomination.
As disappointing as he was though, I honestly don't know if I can blame BioWare for this one, as he seems to be the exact sort of villain I saw people asking for after Dragon Age 2 didn't have one.