I've had mixed experiences at my local one.
I was tempted to do a thread last time I went, to rant about it, but I didn't because I didn't want to put anyone off of donating :(
Time before last a guy shouted at me and the bloke doing the paperwork because he had donated over 25 times (or whatever, can't actually remember or care) and I hadn't, so I shouldn't be able to just drop in and give blood!!!! He had more right than I did to donate yadda yadda yadda... we were both
. I think he just wanted his next little badge 
But last time I was really annoyed. I'd booked an appointment and they were running 1.5 hrs late but shite happens so I just waited patiently (hadn't taken a book or anything as I had an appointment and I'd never had a problem before) so I'd counted ceiling tiles, floor tiles etc.... finally get up on the bed, remind the woman that my vein is difficult but she needs to use that one there - just there... (I've had a lot of cause to know this over the years) - she says 'OK' then decides to use a different vein - it hurts like FUCK, she's missed the vein, wiggles the needle around in my arm, I almost pass out with the pain so she removes the needle. She half mumbles an apology. I say 'It's OK, don't worry - but you need to use this vein - there x' and she says 'No, I can't do it now - I can only attempt it once' ... I tried to convince her otherwise but she wasn't having it.
So a whole evening wasted, not able to donate, because this woman did not listen to what I said. My arm hurt for 2 days and was badly bruised for over a week and I have to go back. I'm going again this week & I'm dreading it... I hope it's better this time :(
However, no matter what I go through, I know it's nothing compared to what someone who needs it has been through :(