Viagra, Cealis and all the rest of the ED drugs really kick ass in the erection department, if you're twenty-something you probably don't need it, but it does definitely enhance your erection, often allowing one to orgasm and continue without losing the erection.
Unless you are twenty-something it probably will make you harder and slightly larger than normal....But it won't make a John Holmes out of you unless you already have a monster cock.
I've found the only side effects I get are the occasional slight headache and some slightly delayed orgasm effects.
As Kpooh said, Pot and Viagra is really mind blowing!
But of course pot is illegal.