As everyone says, neither leather nor kevlar protect from impact. You need armour and padding for that. However, they do a pretty good job at protecting from abrasion.
The only company I know of that makes "kevlar" gear is motoport. (I wonder why...). They actually use two completely different materials which they call "kevlar". The first one is "Air Mesh Kevlar". That truly is a mesh (similar to polyester mesh like Joe Rocket Phoenix). However, it is very stiff and bulky. The other fabric is "Stretch Kevlar Blend". That is not a mesh, but it is fairly breathable (similar to thick denim). It is very soft and stretchy. It looks and feels very similar to keprotec, which is used in stretch panels on leather jackets and pants (e.g. see
here or
here). I own version 1.0 of that jacket, and the stretch panels on the sleeves look *very * similar to the stuff my pants are made of.
According to the company
propaganda "Air Mesh Kevlar" actually has less abrasion resistance than leather or 620 denier cordura. However, "Stretch Kevlar Blend" supposedly has greater abrasion resistance than leather.
This is consistent with the claims I heard about kevlar. It has an incredible tensile strength (i.e. if you take a strand of kevlar and pull it apart, the force required to break it is very high). This is what allows it to stop a bullet. However, it has a rather mediocre abrasion resistance (i.e. if you rub a patch of kevlar with sandpaper, it will not last as long as leather).
The stuff that is typically used for motorcycle gear is actually keprotec, which is a blend of kevlar, cordura, and lycra. Motoport people don't say this, but I'm almost positive that's what their "Stretch Kevlar Blend" is.
Anyway, last year I actually went down there to check out their wares. They have a small office/factory/warehouse at a plaza in Carlsbad, about 2 hours south of Los Angeles. I ended up ordering their "stretch kevlar" overpants, primarily because I was not able to find any other overpants that are both strong (cordura/leather) and breathable. The choices from other manufacturers were either polyester mesh (kinda weak...) or "waterproof" cordura/nylon (too hot/non-breathable).
About 14 weeks and over $300 later, I got them. Expensive, but pretty well maid and comfortable. They also have long knee/shin armour and hip armour. My only complaint is that the knee armour is about an inch too low. However, I can shift it up when I get on the bike, and at some point I'll improvise something to support it from the bottom. Also, I need to saw the zipper to attach it to my jacket, cause every manufacturer has their own "standard".
However, I decided not to get the full suit from motoport and instead I'm sticking with my leather jacket. It is quite breathable and comfortable due to perforated panels, has very good armour coverage, and you can't beat the price.