Riding a jetbike gives them T4 and 3+ armor, so they are essentially MEQ. However, they are fast and have jink, so are superior to the normal marine. This makes our choices much clearer. The obvious answer is Grav weaponry, but I'm not convinced that we should knee-jerk and take massive quantities of it in our normal lists. Especially since the jetbikes will often out-range these weapons. We have the special issue ammo of the Sternguard, but we face the same problem as Grav. Same issue with plasma. So you will need to ensure that these weapons are placed on mobile and/or relentless platforms who can keep in range of these sneaky xenos scum.
Missiles are an option, as you'll kill approximately 1.11 MEQ per two krak missile shots. You will just have to watch what you put these missiles on, as they will likely see return fire. They will also help against the MC's that your Eldar opponent will likely put on the board. The lascannon will also work here, but will obviously be more expensive to field. The Vindicator is also an option for you, as the Demolisher Cannon is vehicle based, it'll be able to keep up with the jetbikes.
Alternatively, you can use fast, but cheap, tarpits to harass your Eldar foe. If they are in close combat, they can't shoot you can they? It will be tough to accomplish, as you don't get much faster than jetbikes, but not impossible. Vanguard Vets come to mind, as do bike squads. Or really any opportune unit that comes into range.
Honestly, my best response is to field an army that has a mix of long-range weaponry with low AP and fast moving fire-platforms. This gives us some first turn firepower, as well as being sustainable throughout the game. The fast moving units will be able to bring in the mid-range weaponry, which typically is more powerful and able to cut through the 3+ armor. The other suggestion that I would have is to field MSU. These jetbike squads tend to be a little bit larger to protect against leadership checks, so they have to put their shots into a single unit at a time. If you have numerous units, its much harder to eliminate everything with sufficient time.
Do you all have any other ideas or comments?