All they really do is help you get hard. Its not like you're going to be able to go 5 times in a row like some claim.
In my personal experience, the side effects aren't worth it unless you have serious problems getting it up. Headache, stuffy nose, and flakyness with the woodrow. I guess for some guys, the big V makes you hard as a rock for hours - but in my case I had about 15 minutes and then had trouble getting it up afterwards (when without the V, I have no problem). I tried it mainly out of curiousity.
My best luck has been with herbals. Not completely safe either, but much better than V. Much less side effects. There are some formulas you can get that seem to work pretty good. Cobra, Python, Mens Virility Formula. You can score these for 10-20 bucks a bottle (less online) and have decent success. If you take them daily according to the bottle, you'll notice that when flacid you will be bigger (after a few weeks of continous use). They don't make you bigger when hard (but maybe a little harder). These 3 formulas all basically contain the same stuff as those penis enlargement pills (which don't really work, other than making you bigger while soft).
Even if you don't take them for a while, taking any of those I mentioned will help you get harder during activities. I don't take them regularly anymore, but will take a combination of them every few hours before a session - and have had good success. Be careful with them though, especially if you have blood pressure problems. Most of them will raise your BP through the roof if you're sensitive to that kinda thing.