I think colossus' starts to shine more when you get the components involved, because of the sheer amount of armor/shield buff that they tend to give, and the frequency of +health drops. As a colossus', I found that it always encouraged me to be in the thick of things - I prime them, they explode, then everything around us explodes, and then I just keep doing that as I am picking up health drops from those constant explosions.
What becomes trickier is once you're not dealing with mobs that have much in the way of trash around, because suddenly those incidental hits are harder to get rid of - your shield can take a beating, sure, but your health regen seems to require a substantial amount of time out of combat, and you lack the overshield that other classes have to buffer you while you're low and trying to find more health.
I played my colossus' really aggressively, and it worked well. I didn't have a ton of super long ranged weaponry, but I didn't face much in the way of problems. I never did get the firewall mortar to drop, but I was planning to give that +rail gun a try, because I think it would add a LOT of versatility.
In general, I see the colossus less as a 'tank' and more of a brawler - if you get him in his element, it goes great. If you fall behind the curve, it gets a LOT more painful suddenly.