I wear a Hit-Air MLV vest for street riding and think it is great. I absolutely think that are great for the track. I think most if not all the pro racers use them now.
I do believe impact injuries are the leading cause of death or long term injuries. We usually think abrasion resistance, which is import enough to me that I wear Motoport Kevlar gear (mesh in the spring, summer & fall, stretch in the winter). But impact injuries is what I am trying to minimize. In my last low speed low-side, which was over 10 years ago, I injured my shoulder, scrapped up my knee and stubbed my big toe. My shoulder still bugs me, my big toe looks funny (the old nail turned black and fell off), but I can't even remember where exactly I got rash on my knee.
With that said, I do think the Alpinestars system is great, but it is pricey. The Hit-Air is very simple and I got it for 50% of retail from a rider getting out of riding. So it works well enough for me that I won't be upgrading in the near future. I think the Alpinestars unit works better than the tether on the H.A. vest, but the tether would work well enough in most crashes I believe. I may not do as well in a low-side. But I think it would work excellent in a high-side, which is exactly the place I would want it to work. It isolates the head and offers impact protection to the torso. Although, even in a low side, if it deployed a tad late, it will still help with secondary impacts (particularly on the street vs. track).
You may also take a look at the Helite Turtle vest. That is the one I would buy today if I were in the market. It has a slightly faster reaction time and the built in back protector that goes outside the air vest. So it not only offers increased protection, but abrasion resistance for the actual air bladder.
It works well enough, but does want to be in the same position as my messenger bag. The fix is to stop using the bag, but I love it for off the bike carrying of spare gloves, camera, etc. After my last tank bag got stolen off my bike, I'm hesitant to go back to one and keep my spare gloves in them.
It is a tad warmer than riding without, especially in the summer, but I am in Arizona. It doesn't make a huge difference, but it is just noticeable. The vest snugs up the armor, preventing optimum airflow. But I do believe it is worth it for the potential impact protection it offers.
Between the armor & abrasion resistance in my Motoport gear and the Hit-Air vest, I think I am about as reasonably protected as I can be. I also have the vest in Hi-Viz as I used to commute at the time I got it.
