I've given blood three times now. The first time I wasn't hydrated enough and didn't give enough blood and ended up with horrendous bruising all the way down my arm.
The second time was fine - I'd hydrated well enough and had no bruising or fainting issues.
The third time again was OK until I'd finished. Sat at the little treats table with some squash and a choccy biccy and was just thinking it was about time I made a move, when I started feeling all clammy. I think the colour must have drained totally out of my face as the nurse just took one look at me, hauled me up and back in to a chair, swung back with my feet in the air! We did have a bit of a laugh about it as I sat there upside down!
I think some people are more susceptible to fainting than others. A friend of mine only gave blood once as his blood flowed so quickly he fainted and had a fit. He was recommended to not go again!