I have the Mach 5 version. Its okay. Supposedly, it could provide a pretty good aero profile, but there are a number of gripes. Minor, but there nonetheless.
1. The hip strap buckle is off-center to prevent scratching on your tank. Problem is, its really difficult to use and clip in when geared up. I ended up taking it off. This allows it in rare situations to catch some air and lifts it off my back. Not a big deal but it can be a bit unnerving.
2. Its designed with a pass-through for a hydropack or wires, but the opening is inside of the straps (not on-center; and only on one side). Its easy to get tangled up in it if you use it this way and since the slit is only on one side, it may not be on the side you want which could be a problem if your tube or cable is a tad too short.
3. The zipper is on the panel that's against your back. You pretty much have to remove the backpack to access anything. Its inherent of the design, but if you are one of those who has a friend access your backpack while its on, you may not be as happy with it.
4. Stuffing the pack is uncomfortable. Its not uncommon for people to stuff a backpack to the point things begin to bulge. Since the only soft portion faces your back, any bulging will be right into the rider. The Mach 7 can expand a little if you open a zipper. Not sure if the lower models can do that.
5. The helmet carry strap is convenient, but it moves the adjustment strap to a not so intuitive location and its hard to find while riding on that side. It would almost be nice if they made the other side the same way so its easier to remember where its supposed to be. I often and up grabbing the helmet carry strap instead. Mine also keeps coming loose at a certain adjustment point. Not sure if its something that only happens to me or not.
It works fine as a pack, but aside from the cool look and hard shell, there isn't any real standout feature to make it an awesome buy. I'm not going to replace it anytime soon, but I'd look for something more functional next time.