I've messed around with pumping a bit over the years, nothing serious, just for fun now and then. For me, when it comes to changing the shape of the penis from pumping the usual most undesired result is that the top of the foreskin gets really misshaped. From what I've gathered, if you are circumcised, this is essentially the scar tissue from your circumcision swelling up. (Internet science, could be right, could be wrong, I dunno)
The solution I found, if you put a cock ring at the base of your head, then it prevents that unwanted swelling. The end result, for me, is that my cock gets massive for a day or two and then goes back down to normal. haha. The look a pumped cock is a bit different, usually really smooth and not many veins showing because the pump really just makes the skin around the penis swell up and expand from being in a vacuum.
Like I said, I'm not a hard core, dedicated pumper, just pump up a few times a year for fun. It doesn't hurt, if you do it correctly you won't hurt yourself, its had zero effect on the functionality of my penis...all it really does is makes me like 9" in diameter for a short period of time. haha.
Oh, also, for about a few days after it does make your dick look a little odd because when its pumped, your head looks massive and full and huge, but (for me anyway) my head "deflates" faster than the rest of my dick so it gives this disproportionate look to it. My normal sized head, with this super fat shaft. haha.