Actually, those bad reviews don't bother me that much because you can tell straight away to ignore it.
What is worse is when someone raves about something, giving it a 10, and it's not so obvious that they've never really experienced anything better. I could give some examples, especially when it comes to manual lenses, of lenses that just aren't that great but someone's really excited about them. That is why I'm not going to write any lens reviews until I've at least tried some Limited lenses, so I can have a better understanding of what a 10 really is.
Also, I have to say that some lenses are probably better than people give them credit for. The pixel-peepers might sneer at a lens that is not sharp enough in the corners, but the lens might still have beautiful rendering, color and contrast. It also seems like some people reviewing something already have pre-conceived notions about it before they even tried it (this or that coating or lack thereof, for example).
And you can't discount the fact that a great photographer can make a mediocre lens on an OK camera give stunning results. I've seen that recently when seeing some pics taken with a K20D and a non-SMC Takumar budget zoom. I know I couldn't have taken pics as good as those, if I had a K-5 IIs and a Limited prime. In fact, I just found them:
If you go by the reviews of that lens, you'd think it's a paper weight.