I agree with the reviewers that point out the low cut front; its lower than I would design it, but not awkwardly so (for me). There is no belly showing when standing up strait. I think my Hincapie team bibs are still my favorite (incredibly comfortable) but these are very nice. Chamois is great.
Now to sizing: I'm 5'9", 130-135 lbs. 29" waist. I have lean thighs (my review might not be accurate for guys with bigger thighs). I have worn Volero and Hincapie Bibs. I wear small in both (small jersey and bibs). I originally ordered a medium Bormio kit based on the near universal recommendation to size up. However, with the exception of the Bormio long sleeve jersey, the mediums were all too big for me. Now wearing smalls. The small bibs (which you're looking at here) are admittedly tight, but I had no problem getting in them and they broke in beautifully. Shoulder straps fit snug but are not awkward or constrictive.
In short: if you are used to European style, aggressive cycling sizing, Castelli (including the Bormio) will probably be more or less true to normal sizing. If you normally wear small but are on the border of going medium, you probably should go medium. But if you are normally right in the middle of small, you'll still want small here. It may admittedly be a bit tighter but not enough to have you change sizing.
And the kit, including these bibs, looks great. I get compliments. Full retail? I don't know... but bonktown pricing was a steal.