Having spent a brief period of time working in one of these facilities - if you're going to do it, make sure you pay attention. The phlebotomists that most centers hire are typically the bottom of the barrel, so to speak, or fresh out of schooling (if they've even been professionally trained). They also pay these people sub-par wages (I know).
The thing with plasma donation is that they are taking blood out, spinning it, siphoning off the plasma, and then putting the red blood cells back in. This is where paying attention really matters. If the vein is perforated (and it's a big fucking needle that is used), when the RBC's are being pushed back in, they'll start to just bubble up under skin, leaving a terrible bruise.
Plus, as previously stated, you're a fag. We're not allowed to donate, silly.