My impression of the Colossal is that it's more intimidating then actually effective. It tends to draw a lot of fire, leaving the rest of your forces to do their work. I only played two games, so it's a small sample size and as I haven't played in a while it's probably an inaccurate analysis.
The first picture is from my first game, against Menoth. We played about 5 turns and it took over two hours, so we ended up calling it. My opponent thought I would win, though I wasn't sure, but I wanted to get at least one more game in so I agreed. My goal was to get back into playing Warmahordes, so it didn't really matter to me what happened.
My second game was against my friend who taught me (and is still teaching me) the game. His Mercenaries list is brutal, with lots of knockdown attack and ways to punch your caster in the face. Great list played by a great player.