Space Marine Bike Squad
Bike squads are a tough, flexible choice. They’ve got better firepower than a Tactical squad with twin-linked bolters and two special weapons, though they are limited to a multi-melta or heavy bolter on a single Attack Bike (which they should always take). Bikes are a medium range firepower unit, and should use their speed to target vulnerable or high value targets. Make sure to keep them out of assault; while they are tough to kill, they won’t do any damage either. Their value is in shooting, so don’t let them get tied up unless you really need to tie up the enemy unit.
Bikers can also be taken as Troops if you take a Captain on a bike as an HQ. This gives you essentially fast Tactical squads as your Troops, which make for a fun and useful (though small) army.
Attack Bike Squad
If Bike squads are a faster Tactical squad, then Attack Bike squads are more mobile Devastator squads. They can take either heavy bolters or multi-meltas, making them great at gunning down infantry or hunting tanks. They’re fairly durable and can apply their firepower pretty much anywhere they want to, making them great choices for the points.
I particularly like them armed with multi-meltas, as fast and reliable tank killing is hard to find in a Space Marine army (we’ve already covered why I don’t like Speeders for this role). They’re perfectly serviceable with heavy bolters, but I can think of lots of cheaper ways to get anti-infantry firepower. With the range of heavy bolters, you’ll rarely need the mobility of the Attack Bikes.
Scout Bike Squad
Scout Bikers are one of the more mobile units in the game, combining the speed of bikes with Scoutsand Infiltrate. However, with only a 4+ save, Scout Bikers are much less durable than Marine Bikers. That leaves us with a squad that specializes in positioning and mobility but must be very careful in its placement if it wants to stay alive.
If all Scout Bikers could do was get into position, they wouldn’t be much use because they’re not scoring. However, they do have the capacity to do some serious damage. While they can’t take special weapons except for a combi-weapon on the sergeant, they can take up to 3 astartes grenade launcher. These give them rapid fire S3 blasts or S6 shots. At close range, these can do a lot of damage to both infantry and to the rear armor of tanks.
One of the reasons outflanking units have fallen out of favor is that they can no longer assault when they arrive. This hurts most outflanking units, as they’re assault units. Scout Bikers, on the other hand, can arrive and put out an impressive amount of firepower, enough to neuter most enemy backfield units. I would generally keep a squad small, probably 4 models with 3 astartes grenade launchers and a combi-flamer or melta.
Finally, Scout Bikers can take cluster mines, which allow them to booby-trap one piece of terrain. The first time a unit (either friend or foe) moves into the piece of terrain, it triggers the cluster mines and take 2D6 S4 hits. These are unlikely to do much damage, but they’re cheap and amusing. I say take them if you’ve got the spare points, but don’t sweat it if you don’t.