I donated plasma A LOT in college for some easy cash while I studied.
Pros:
-You sit for about an hour and get paid to read, study, listen to music, netflix, whatever. My place had free Wi-Fi.
-Sometimes they do decent monthly bonuses ($100 in just two donations!) and raffles so you can come away with something cool.
-You're saving people's lives.
Cons:
-It hurts a little every time, more than donating blood. Hurts really bad when the newbie misses your vein or goes all the way through it. Remember, they have to stab through the same holes as last week, and they aren't fully healed yet.
-You get permanent scars that make you look like an ex-addict.
-The standard rate (at the time) was $25 your first visit, $35 your second. You had to go twice a week to get the max.
-It really does take a lot out of you. You have to drink tons of fluids and hearty meals to make up for the donations you're giving. If you don't or forget, your next visit will HURT, and take forever (once took me 2 hours), because your veins are contracted from dehydration.
Overall, I got enough bonuses to eventually buy whatever stupid gadget I wanted from amazon and it wasn't coming from savings. But I don't think I would do it again, because the hourly rate is only decent, and coupled with the effort and mild continuous discomfort, I can find better ways to help people.
However, if I found out I was a perfect match for someone, or became disabled and found this was the best way for me to help people, I would do it again.
*ETA, If you donate blood, use any kind of drugs, or cannot pass a physical, you cannot donate plasma.*