Bikers are a funny unit to try and use. You'd think it was easy with a superfast ork on a bike

Though really they've got their uses in two main ways.
First is the Wazdakka army. Bikers galore as your troops, using the Fast Attacks for the koptas or buggies. Big units for much killing.
The other is in a standard Speed Freakz army, which is what I use. Two squads of 6. May seem small but outside a wazdakka army you have to consider the footprint of the unit. Too big and the unit is screaming for it to be shot - it'd be pasted in a Turn or two. Too small and they'll get pasted faster because there isn't enough models. So about half size is a nice unit.
In both cases,
PK/
BP nob leaders in the squad and you'll be good to go. Throw it at infantry or a tank and it'll let rip. And those dakkagauns are mighty handy so try and get a volley off before you charge.