I had been riding in 315's for almost two years. The 315's were legendary (and a favorite in the pro peloton). I didn't think they could be improved. They have been! And not just by a little.
The new 320's are significantly lighter, feature a better heel cup gripping system, better walking pads (beefed up in the rear), the Dynalast system has been fine tuned, and the sublime hollow-channel, cross-weave carbon fiber sole is as light and effective as ever (now in matte carbon). In addition, and this was key... the structural support/design along the interior arch has been much improved -the only real weakness of the 315's.
It is two other features, however, that set these shoes apart from the rest and makes them really special.
(1) An unrivaled Ventilation system. The 320's feature a metal mesh "grill" in the front combined with breathable inserts throughout the upper (and one in the sole) to facilitate flow-through ventilation. This feature keeps your feet drier, cooler, and less clammy. It makes a very noticeable difference, especially during hard riding even in mildly warm weather.
(2) The 320's also feature a very roomy, wide toe box so that you can power "down" without constriction or pinching of your toes or forefoot. I can wiggle my toes fully; yet the heel and midfoot is totally locked in. Thus, I can power down flat for full power. And my foot is cooler and drier. Fantastic.
Let's face it, a top-of-the-line road shoe from any of the Big names is usually a really good shoe. But, the fact is there is -nothing- out there quite like these Shimano 320's. The ventilation system alone makes them unique, yet it is the synergist function of this shoe as a whole that brings it into the realm of PERFECTION. ... If Shimano 320's are in your cycling budget, buy them. You won't regret it.
FIT: Shimano 320's are certainly wider in the forefoot than regular SIDIs. The 320s will fit an average to slightly wide foot very well.