Head injuries suffered by motorcyclists are a leading cause of death and disability, and are proportionately more severe than those sustained from other road traffic or sporting accidents. The specific types of brain injuries following motorcycle accidents tend to have prolonged consequences and often incomplete recoveries. Your helmet is important.
Motorcycle helmets do more to protect us than any other piece of kit – they reduce the incidence of death and head injury, reduce hospitalization, and reduce the chance of intensive care admission. The overall head injury reduction risk is about 72 percent; well worth a few extra thousand Baht invested in a good motorcycle helmet. It's worth noting though that you have to invest to get the benefit, or you might as well not bother. If you compare study outcomes from countries with 'type approval' law and countries without type approval law, riders with non-type approved helmets actually sustain head injuries more frequently, and of greater severity, than those riders who crash with no helmet on at all!!!
Cheap and heavy motorcycle helmets are a bad investment – not only are they less effective at dissipating the force of an impact, the extra weight of a heavier helmet on your head has to pivot somewhere, and this puts additional strain on the cervical spine, increasing the likelihood of a neck injury. Good motorcycle helmets look awful after a decent crash. But then, that is the point. A motorcycle helmet is essentially a big crumple-zone with an inner safety cell, much like modern cars. If it has had a good impact, it should look like a mess, as the forces have been dissipated over a wide area of the helmet's shell. The top of the range Arai, Shoei, AGV and Shark helmets in particular look appalling after they have done their job correctly. Conversely, I have seen several 'budget' motorcycle helmets belonging to severely head injured riders with scratches that would butt out.
Worldwide it's common practice to transport a rider's motorcycle helmet into hospital with them – it serves as a useful guide to where the impact occurred; but they are a poor indicator to how severe the head injury is – a smashed and dented helmet is often a good thing.
One note on chinbars – n my experience a disproportionate number of incidents involving disruption of the helmet chinbar results in the rider being critically injured, since we find the rider almost invariably has a compromised airway as a result.
Another crucial aspect of a good motorcycle helmet is its ability to stay on your head. Sounds stupid, but in the bad old days we'd occasionally see a helmet come off completely when the lining was torn out in a big impact. The danger still exists if you are riding in a helmet too big, or with worn lining, or a less secure 'chin-belt' fastener.
The final word is take your motorcycle helmet purchase as serious or even more serious than the way you bought your motorcycle.