Not bad. Be there at eight when they open if possible - I waited about half an hour to be seen and the fluorescent lights were making me feel sick, especially since I had to fast 12 hours ahead of time (which became 13 hours). The signs in the office all say to keep pressure on the site for 1 minute afterwards, but you should actually do that for 20 minutes and not lift anything heavy for 24 hours, which they don't tell you.
On the other hand, the drawing process was very easy, just a tiny prick and then no pain during the process, and the staff were all very friendly.