I love mine. Can't get enough of it. I don't think I could get away with wearing it over a chest protector though. At 240 it's just about at its max for me but still roomy. I do wear it under my Klim Valdez at times. There's just so many pockets, D rings, etc. to put stuff (cam, energy bars, tools, tubes, MSR fuel bottles (yes I know), etc.) not to mention water bladder system. Another great benefit is the fact you can load it down and it'll feel pretty heavy but once it's on it distribute's the weight very well. Bottom line is I don't go on long rides without it.
Depending on your size you might be able to pull it off. Hopefully others will chime in.