I've got the 58 and I weigh 165. It's plenty stiff for me, and I would put it on par with a stiff steel bike. The downtube is definitely...oversized. I love mine -- it's one of the best value in road bikes.
Can some one confirm that the geo is not racy ? I am looking for more of a upright position as I don't race. Also, can I put in some spacers if needed to get a more relaxed geo ?