I have become a platelet donor now. Fits in with my lifestyle better. They hook you up to a machine for longer (it's about 75 mins for me) and suck blood out, centrifuge the platelets off and the pump the blood back in. The good thing is that you get all your red blood cells back and so you can still train/exercise and you recover more quickly. You can also donate more often. The same safety screening checks still apply.
If you are A pos (like me), they are particularly keen to recruit. I don't know why but A pos is like 'O neg' for blood - universal donation. They will take anyone by the way so if you are not A pos, don't be shy!
The downsides: the procedure takes longer, it is more uncomfortable (as the needle is in longer and your arm is in the same position for over an hour - they do give you a local anaesthetic to help), and you have to make sure you have plenty of calcium or your lips start tingling and they had to give you a glass of milk!
I'll admit, it's not for everyone but for those of you who do donate blood and feel very fatigued afterwards, or if you do a lot of exercise, you should consider platelet donation.
Platelets, amongst other functions, are critical in the complex blood-clotting chain and in the inflammatory process. Where do your platelets go? They are used mainly in people who are immunocompromised (chemotherapy patients, etc) and in very premature babies who cannot form their own platelets.