Make sure you get plenty of water, square nutritious meals, and ample sleep for the 24 hours leading up to it. If you're in starvation mode or have low blood pressure or anything is out of balance, it might complicate things.

You will be standing in line (sitting if you're lucky) for a while, and you will be flexing your muscles several times a minute with a needle sticking into your arm (while reclined), while keeping with a set of instructions. The whole thing will take 1 hour at the shortest, excluding travel. So it is certainly labor, but it will deplete your body more than labor does because you are losing maybe 1 or 2 days' worth of biochemical synthesis, and it increases the load on your cells to make up for it.

You may be able to make $200 or $300 for your first however-many donations in a 3-month or 6-month span. I wouldn't do more than that. Any more frequently will lower how much they pay you per donation, and you don't need to devalue your body/time by taking anything less than the best deal possible. Personally, if I was given the choice between working longer hours, taking on a second job, living on rice and beans, and giving plasma, plasma would be my least preferred option. YMMV though, every body is different. I would recommend going and doing it once, so you can either say "been there, done that" or you can see if it fits into a short- or mid-term financial strategy.

I've known plenty of people to do it, some of them do it for weed money or financial breathing room. I've even known people with fulltime jobs that do it every so often. I've never known anybody to do it consistently enough to pay rent or anything.