I loooooove my corys with my betta, and he loves them. I think they're perfect. The ones I have aren't super-fancy (which I think is good because my betta likes to be The Fanciest One). I have C. trilineatus, the ones sold as "julii" corys or leopard corys, and they have cute black and white spots and black fins, so they may be among the flashier of the corys.
They have to be in groups of say 4+, but I just added 2 more to my 4 and they all just blossomed socially. They're totally awesome and you can just watch them forever. They totally steal the show from my betta. He'll all posing and trying to be sexy with his glamorous fins, and they're wiggling around on the bottom acting really goofy snouting around in the gravel with their ridiculous barbells and taking all his attention away. They're cool, if by cool you mean geeky.
And, amazingly, they are also buddies. I've caught them several times playing together in this one spot of filter current. They take turns riding the filter up and down the glass. It only happens in the evening (like around now), and it's hilarious, because the corys'll be doing it with their little catfishy bodies and then the betta will come in and it'll take him like twice as long. It's like watching surfers and a sailboat together. And I don't think the betta would ever have done it if he hadn't seen the corys doing it.
It's a perfect mix.