I recently read of a man in Thailand who'd had a penile injection (far too common for comfort among Thai men) to increase the size. One of the usual ingredients that is injected is olive oil. (I couldn't say if he ejaculated salad dressing or not). So, eventually he developed a really aggressive cancer and had to have the thing removed.
So, I suppose the moral to that story is something like - are the risks worth the benefits?
It seems to me there would be three main reasons for a guy to try enhancement.
1) To increase his self esteem.
2) To attract more women
3) To better satisfy a woman
In relation to No1 - Just buy a Shelby Mustang. A cool car has been supremely effective since car's were invented.
For No2 - See answer No1
And No3 - Well, No3 isn't an issue. If she's not satisfied with your size, let it be her problem.