I wrote a review quite a while ago about the bibs I own and use. I've another 6 months using the bibs this season, and after A LOT of mileage on some of them I think I can offer far better feedback which hopefully someone out there will find useful.
The bibs I will comment on will be Hammer Nutrition '08 Voler, Assos S5 FI.Uno, Gore Xenon, Giordana Forma '08, Performance Ultra, Santini Gel Twist '08, and Sugoi RS '08.
In order from worst to best.
Hammer bibs: These are great bibs. I think for many people they are a great deal, and should at the very least be tried because if they work for you you'll save a ton of money. They don't work for me, unfortunately, being too loose in the butt area. I get friction, and even for commuting I find them uncomfortable. It's a real shame because the fit and material is excellent everywhere besides the butt. I think the chamois is a bit flimsy compared to my favorites, and the material covering it is not nearly as soft as my favorites. The shoulder straps are a bit too stretchy, but still adequate. I like the mesh uppers. They are cut reasonably low so you don't feel constricted. In summary, I almost never wear these anymore because they create friction which in turn causes saddle sores. I simply can't deal with the sores due to the mileage I'm putting on the bike. Very disappointing!
Performance Ultra: The only reason these are above the Hammer bibs is because they don't cause discomfort. The bib short material itself is nice. I really like the eschler fabric. The leg openings are very tight compared to all my other shorts, but not so tight they create discomfort. I do notice them every time I use them though! The chamois is pretty much a joke, in my opinion. If you like very flimsy, thin chamois' then these are the bibs for you. Or, still, maybe not for you. The chamois cover is soft so it has that going for it, but it offers no padding whatsoever. The shoulder straps may as well not even be there. They are so weak they almost fall off my shoulders. No other bibs I own come close to being as stretchy as these. The fit is also not ideal. They do not conform to my body perfectly, notably in the crotch area. Overall, they don't cause any issues, but they just aren't as refined as the significantly more expensive bibs I've come to enjoy. Perhaps that last sentence is so obvious I should delete it! I use these for commuting when I can't find anything else to where. One last comment, I don't think these are very durable.
Santini Gel Twist: These bibs are really expensive retail. Thankfully, I did not pay the full retail price. The material used throughout is excellent. They do look and feel like very high quality bibs. The gel chamois is not impressive. It's pretty thin, so again, if you are in to the thin chamois, these may be worth considering. It crackles like a diaper some times. It moves a bit like the Hammer bibs, but not so bad it creates discomfort. The gel doesn't seem to do much, but the chamois covering is nice and soft just like my favorites. There's a tiny little pocket in the rear which might be useful for some. I've never used it. Keys or a small mp3 player would fit. Or a credit card, etc. These, like the others above, are for commuting only.
Gore Xenon: These were my first high end bib purchase, and I thought they were the greatest thing since cinnamon raisin peanut butter, but after gaining significantly more experience, it's obvious these are more like Wonder Bread. The fit is top notch. The compression is equal to my favorites. The shoulder straps are nice and snug. The cut is very low so they really feel great on those smoldering hot days on the ashphalt. The chamois is well thought out, reasonably soft, and offers just the right amount of padding. Almost everything is proper with these shorts. So why are these ranked so low? The durability sucks. They are falling apart, and tomorrow I'm going to call and see if I can get a warranty replacement. If they have the tags I'll sell them on eBay and make some $'s!
Giordana Forma '08: These bibs, like the Santinis, are ridiculously expensive. They are my most expensive commuting short I use, and have the added distinction of being in the 'desperate training bib' section when all else is dirty. I find everything about these shorts to be about average to above average. Bibs are very personal, and I think these are probably the best for certain individuals. The chamois is pretty thin, and has not much density to it. It's cover is very soft. The leg grippers are the best of the bibs I own. They simply disappear and don't leave a nasty impression after wearing them. The shoulder straps are also very nice. They are snug and wide, and fit perfectly. They are probably the best shoulder strap of any bib I own, but my favorite bib's straps are just as functional. The fit of the shorts is very good. They are very well made, and the fabric and stitching is durable. I think they are slightly more elastic than my favorites, and do not offer as much compression. The size is right, so it's not like they are too big. Overall, if you like a thinner chamois with a medium-low density these may be just your bib. Definitely worth checking out!
Sugoi RS '08: The what could be the best bib I own bib. But they aren't. Like the Xenon's the durability sucks. They have been used way less than the Xenons and are in even worse shape. I like the chamois a lot thus the only reason they are ranked this high. Everything is perfect about this bib besides the quality. The stitching is unraveling in numerous locations. A real shame, because I would maybe get more of these if it weren't for this very serious problem. The chamois is very dense under the sit bones, and well cut throughout. It's not as soft as my favorites, but thus far this has not been an issue. The fit and compression is excellent. Just that darned durability... I'm going to try and warranty these tomorrow as well!
Assos S5 Fi.UNO: The go to bib. These are the best bibs I own. Luckily, I have three of the exact same. Compression is perfect. The chamois is super soft and has all the right density and cuts in the right place. The cut is low in the front and sides. The rear breathes incredibly well. The silicone dots for the leg grippers work as they should, and are not as obtrusive as the others. Note: they are very similar to the Gore Xenon. A simple cut to the very high quality fabric clearly shows you don't need 14 million panels to make a perfect fitting bib. The shoulder straps are very snug and even make me hunch over a bit, but that all goes out the window when I hop on the bike. These things become one with my body! If you can only own one bib these should be on the short list.