I did it for about 5 years.
Depending on the company you are working with, and how busy they are and the current demand (money you get), it CAN be worth your time, or maybe not.
I watched it go from 20/35 to 35/75 for payouts in my 5 years. with crazy perks, like after not missing a donation in 12 weeks you get a 200$ bonus. Or doing the booster tetanus shots every month for an extra 10$ per a donation.
I have also watched waiting times go from 0-60mins and during certain times up to 120 mins! Just to be seated....
If the math worked out to be around 20$/hr i would still do it, but due to the location of the place, the and wait times and current payouts here, it isn't worth more than 15$/hr for 4 hours a week.
Also, it stops you from donating blood/whole blood. And it gives you NASTY scares for the rest of your life. :(
edit:
I started with a local small in Austin, TX. Who then was later bought out by BioLife.
Year range was 2005-2010~
ALso checked out DCI plasma and uhhh another company.
And if you really wanted, you could really donate plasma to a blood bank, without receiving cash, that plasma is actually used to help people directly and not to fund research or make meds which are sold for profit (not that there is anything wrong with that)