Reavers have that scary 5+ save everyone is afraid about. In close combat they look like they will get slaughtered, and have a hard time doing much of anything. So I hear a lot of "they might get one shot off before they die horribly to bolter fire or close combat". Well, lets leave noobie land, and get over to how reavers are supposed to be used.
Reavers are able to turbo-boost, and they get an amazing 36" to do so. When turbo-boosting, Reavers draw a line from their starting point to their finishing point. A single non-vehicle unit under this line is hit with D3 S4 AP- hits per Reaver. An upgrade on every third Reaver allows you to get D6 S6 hits. The unit is allowed cover saves as normal. However, if you are simply flying over a unit between armor, or just hidden and not in area terrain, they wont be receiving a cover save. So a unit of 9 Reavers would be delivering 3d6 S6 hits and 6d3 S4 hits. Averaging around 9 S6 hits and 12 S4. These hits are resolved immediately.
Don't forget that this is done during the movement phase. That means doing 25% casualties, combined with tank shocking raiders, can break units off the board rather quickly. If you do manage to break a unit during the movement phase, don't forget to shoot them and cause another 25% casualties so they will run again during the shooting phase without a chance for another moral check. Yea, that means they all just ran off the board.
Now that you've done your turbo-boosting, during your opponents next shooting phase, you get a 3+ cover save for turbo-boosting. I call that pretty durable. Yes, you might be vulnerable to assault, but don't forget that you get 36" to work with. If you came out of a Webway portal, you now just struck anywhere you wanted, and reached anywhere you want.
Reavers should be turbo-boosting most of the game. You do however have some great weapon upgrades, Heat Lances or Blasters. This makes your Reavers highly vversatile, able to break heavy armor and light infantry with amazing success.