I've used both Viagra, and sildenafil. I have medical conditions which means I can get them on prescription free. My GP told me that the ingredients are exactly the same in both, it's just that the drugs company that make Viagra have now lost the patent, so it's now all down to competition.
Personally for whatever the reason, I am finding sildenafil better than Viagra, as I used to get really bad eye strain when taking Viagra, but not with sildenafil, and they work just as good.
I get them in blister packs of 4x 100mg, which I'm only allowed to take half each time, so I break them in half, and take it 1 hour before a punt on an empty stomach, and it really does the trick for me.