I feel a bit weaker in my arm after blood donation. Also my vision can go dark for a moment, if I quickly stand up. Other than that, normally I don't experience any side-effects.
Now I'm donating mostly plasma, and I try to give it every two weeks. i'm "speedrunning" donations, because after donating equivalent of 6 litres of blood in Poland, you can for example have priority in queues in pharmacies, and skip long wait standing behind old grandmas.
Bureaucracy ruins everything.