I worked @ a plasma center for about 8 months performing physicals & doing veinipunctures...
Upside:
You get paid $
It can go very quickly if you drink lots of water as mentioned earlier.
You'll get a free test for lots of diseases, just never tell them that's why you're donating.
Downside:
Scar tissue can be an problem, I can always tell plasma donors, they usually have a punched out scar in the anticubital fossa. Most folks look @ the scar & think you're an IV drug addict (these people have never seen a real IV drug addict's scars)
You can have a seizure while donating or shortly afterwards due to electrolyte imbalances. (I saw several hundred seizures, more than most doctors see in their lifetime)
Drink lots of water & be sure to visit the restroom before you donate, it can be painful to wait (takes 15-30 min). I donated maybe 20 times while working there. It works best to start drinking water the day before so your tissues are saturated. Don't drink caffine containing beverages like coffee or tea or caffine containing soda, before donating, caffine's a diuretic & will dehydrate you.
One of the many reasons I quit working there (actually walked off one day) is that I knew of two 20 year old donors that had strokes who were regular donors. I suspect the hemoconcentration that resulted from the donations led to the strokes, but they are not reported to the FDA as it's ancedotal evidence & doesn't happen immediately.