Some more feedback after doing the equivalent of one heroic life.
There is no indication of whether the champion has the "dangerous" buffs rather than the ones you can safely ignore, i.e., Fort Bypass plus Second Wind, except by examining their info window. This is annoying. That, when it activates, Second Wind has an animation/sound effect for it is good: it gives perceptive players a warning to dodge out of the way.
Most champions' champion status is completely irrelevant as they are no danger or challenge. Here is a picture of Enemy Within that I did today. This room has a bunch of skeleton archers which are required kills to advance the quest. Three of them were 1500 HP bags of sit-here-for-a-minute-and-beat-on-me.
I've seen others mention that they dislike the additional DR or resistances of champions, and I'd like to reiterate that. I already carry a golf bag of weapons/spells for all occasions. The choices I have are to have a random monster suddenly resist a big chunk of the DPS from my custom weapon (while having inflated HPs!), use a subpar weapon, or increase the weapons I carry threefold. None of these choices are appealing.
While most champion buffs are negligible, some are completely overpowered in combination with other abilities. I'm unsure whether it was a DoT effect or increased spell damage, but there was one time I almost died due to an effect like Burning Blood, which did both recurring fire and acid damage. It was enough damage that it went through my resists, and made it unlikely for me to make the concentration checks to pop off a Heal scroll. If someone hadn't been wand-whipping me, on top of my own wand-whipping and pot-chugging, I'd've died. Minus walking around permanently with Protection from Elements, I don't see a way to prevent this; randomly dying to super powerful attacks with no warning doesn't seem fun to me.
I also did a run of Spies in the House where one of the water elementals had the extra force damage buff. Its Frost Lances, even though doing base zero cold damage due to saves and resistances, came with extra, (almost) irresistible force damage tagged on it. Its aura also did extra force damage. Due to its position on a ledge and champion Deathblock, the party serially wiped until someone got close enough to CC it.
These two stories illustrate a point I'd like to make: if you want to challenge people, that's fine. But challenge should be consistent. A quest should fairly uniformly difficult with respect to champions. It should not be a cakewalk if you're lucky with the buffs, or almost a guaranteed wipe when you're not.