I happily did it for years usually going to every session and not noticing much in the way of after effects - usually just took it easy for a couple of days as a precaution. About two years ago I started finding it left me feeling totally wiped out on the day and then took me longer to recover. I carried on donating but found my times were plummeting dramatically; there were probably other factors involved in this as well but, looking back, I'm sure blood donating was also having a fairly major effect. I now just do it once a year and am feeling much better running wise. (I'm only just above the lower weight limit so possibly this has something to do with it, also am more or less vegetarian. Have never had a problem with my iron levels however, and my weight hasn't really changed since I started so not sure why it should have started affecting me so much now - maybe its just part of the ageing process!)