I think just as with any medication it will affect each individual differently and it all depends on your physical condition, age etc I have also found that what I have had to eat recently also matters. I find an empty stomach achieves the best results. I have to agree with othellos comments. I thought I was going to collapse and die when I was book browsing in Waterstones, that was after a punt!
I used to get it on free prescription from the Army Doc, he gave me 100mg tabs and advised to cut them up. Ive found this works well. The civy doctor now prescribes me the larger size as its more cost effective. As another poster has mentioned about coming early in the punt and then having another pop later on. I find I stay hard for 6-8 hours. And the response time can be instant, I was standing in the tube station and became aware that my lightweight Craghopper trs were straining as the desired effect was clearly showing and I was getting odd looks. Again different people react at different times and i think there is a difference between V and Cialis tablets.
As for side effects I get the red face flushes, thirsty and I tend to fall asleep on the train or tube on the way home. I think in some areas of the UK you can get a free prescription and people with diabetes (dont know which type) and also those with hip/pelvic injuries also qualify on medical grounds.