They're fast, they have the blessings of the dark gods, and they can take a pair of special weapons. Chaos bikers are appealing on a number of levels for the Chaos General as they provide a (cheaper than previously) way to bring some fast pain to requisite targets.
As with other chaos units, one of the bikers is a chaos champion and will have to issue challenges in melee, so its best to equip them for that role as they're rather likely to get stuck in to a unit sooner rather than later. The champion has access to the regular ranged and melee weapons, the gift of mutation, and melta bombs. That provides a nice degree of customizability that is comparable to ground based troops.
The squad as a whole can take the usual four marks, icons and Veterans of the Long War. The mark of Khorne and Slaanesh will help with a combat orientated biker squad; whilst the marks of Nurgle and Tzeentch helps with survivability. Indeed, it is worth noting that for Nurgle, a chaos bikers' toughness goes up to 6 which is very significant.
Before looking at some example squad set-ups, I want to make one additional note. One way we can think of bikers is that we get a bunch of marines with a free transport. Let's suppose that we try to construct an analogue unit that is made up of a rhino plus some chaos marines. A chaos marine armed with bolter, bolt pistol and close combat weapon is 15 points. Three of them are therefore 45 points. Adding a rhino takes the total up to 70 points. Hence chaos bikers are completely comparable in cost to (an impossibly small sized) squad of ordinary marines in a transport. But the bikers have advantages: their twin-linked bolters are strictly better than regular bolters. They sacrifice the security(?!) of their rhino for enhanced toughness, so they are in some ways more vulnerable. Never forget they can still die to bolter fire. Use them wisely and use cover to protect them until the alpha-strike manoeuvre that you're planning is able to be executed!
3 Chaos Bikers, 2 Plasma Guns, Icon of Vengeance (125 points)
An harassment unit that should be used to ping high value (high armour save) targets as often as possible. Swap out the plasma guns for meltas to taste. Use their speed to keep shooting pressure on. Use as a lure. Use in the late game to contest objectives. Use two or three of these squads in one army. Job done.
6 Chaos Bikers, 2 Melta Guns, Mark of Slaanesh, Veterans of the Long War, Icon of Excess, Biker Champion with power weapon, melta bombs and combi-melta (233 points)
Take out a transport (even land raider) using the triple meltagun threat. Use the high initiative to get the drop on the contents of the transport (especially enemy space marines) and the champion's power weapon to assault them with. The Icon of Excess will help to keep them alive against the enemies attack and can help to tarpit opponents until backup arrives, if necessary. Team up with a Slaaneshi sorcerer for added fun.
7 Chaos Bikers, 1 Flamer, 1 Melta Gun, Mark of Nurgle, Champion with Power Fist (232 points)
A generalist squad -- think of them as a pseudo-analogue to a plague marine squad but faster! But with toughness of 6, these guys are even better since they're much harder to wound. Opponents are going to be targeting their plasma weapons on them to get them down. They're therefore a distractor unit, so we'd better have some secondary unit of dread waiting in the wings.
8 Chaos Bikers, Mark of Khorne, Icon of Wrath, Veterans of the Long War, Champion with power weapon and melta bombs (234 points)
The anti-infantry squad. Charge!