Ok, I've done this for almost two years, so I'll give you the lowdown.
The requirements:
Weight more than 110 pounds. Not have any blood born diseases or blood disorders (AIDS, amenia, etc). Not have donated blood in the last two months. There are a number of behaviors that put you at risk for AIDS that will defer you. Doing drugs w/ a needle, having sex w/ a prostitute or another male, getting a blood transfusion or medical work while in Africa, and a few others. Basically, you have to be healthy. You are required to have a physical every year or if you stop donating for...3 or 6 months, I forget.
Also, you can only donate twice in seven days and the donations must have a day in between. It's usually best to find a schedule and stick to it. I do Tuesdays and Thursdays. I say "seven days" because it is seven days, not a calendar week. You can't donate Tuesday and Thursday of one week and Monday and Wednesday the next. Tuesday, Thursday, and Monday all fall in the same seven days. Just stick to a set schedule and you'll be fine.
The process:
After the initial appointment for a physical (so they can determine the above) your average day will go something like this. Show up, get weighed, and have your finger stuck w/ a needle. They take a drop of blood and run a few tests on it (make sure you're hydrated and your protein is high enough). Then you have a quick medical interview where they check your vital signs and ask you medical history questions. They ask you the same questions EVERY time. Are you healthy, have you ever taken the following prescription drugs, are you pregnant, have you engaged in the following at risk behaviors for AIDS, etc. This part usually take 15-30 minutes, depending on how busy the center is.
Once they have an open machine they call your name and get the process started. They sit you down in a nice semi-inclined couch next to the plasma machine. They'll disinfect your arm then stick an IV type needle into your arm. This is hooked up to the machine, which will draw off about a pint of blood each cycle. As the blood runs through the machine the plasma is spun off in a small centrifuge and runs down into a collection bag. After a full pint is removed the machine runs the blood back into you along with a small amount of anticoagulant to keep the needle from clotting up. So you are never missing more than a pint of blood at one time. Your body keeps function fine and IF something goes wrong you only loose a small amount of blood. This has only happened to me once and I was fine. It's a very safe procedure.
The cycle repeats roughly 10 times until the full volume of plasma has been spun off. The actual amount dependes on your body weight.
<150 pounds: 690 mL
150-190(?) pounds: 825 mL
>190(?) pounds: 890 mL
When the process is done they run a bag of saline solution into your arm so your circulatory system hasn't lost any VOLUME, just the trace proteins in the plasma. Of course you still have all your red blood cells, white blood cells, etc because those are returned at the end of each cycle.
This usually takes less than an hour. I don't think under an hour is a realistic estimate for the whole visit from start to finish, but definitely less than two all together.
The side effects:
I have experienced very few side effects. I've never had to go to a doctor or something as a result of donating plasma. They do say it can cause fatigue. I'm not sure on that score. I have been fatigued, but I go to school full time, work one (or two) jobs besides plasma, plus I'm in tae kwon do. So you can take that w/ a grain of salt. If you are battling exhaustion or sickness, plasma probably isn't the way to go. If you are generally in good health you should have no problems. There are many days where I've donated plasma, jumped on my bike, sprinted to the bus stop, worked, biked over to get my car, driven to tae kwon do, biked back to my dorm, and been fine.
The money:
Generally, you will get a slight bonus for your second donation of the week. At my place (Biolife Plasma) you get $20 the first time of the week and $30 the second, for a grand total of $50 a week. Which comes out to $200 a month.
The bottom line:
I've been very satisfied w/ donating plasma and have had no problems even after two years of donating. It's a great way to supplement your income on a flexible schedule.
I have noticed that they have to slow down the rate at which they draw out the blood because my vein walls tire slightly and can't hold up to the suction quite as well after time. That hasn't affected me otherwise physically. The only reason I know is because they have to slow the machine down, but I figured I should mention it.
Anyone done it before?
:banghead: 
, I really don't like the idea of anything being put back in me 
The saline they give you is to replace the volume you have lost in your system and is perfectly safe. I suppose your body could deal with that loss on it own (doesn't the spleen have something to do with it?) but the saline makes it easier for your body to get things back to normal.