What I will say if you ask me to read your unpublished manuscript.
If you ask me politely, I will — regretfully and politely — say No.
If you just go ahead and send me your material, without even the simple courtesy of asking me before doing so, I will also say No — but I will not bother being polite about it.
For the polite people: I know it’s tempting, but please consider the fact that reading, evaluating, considering, and critiquing manuscripts is work. I already have a job. It’s writing! If I don’t do my job, it doesn’t get done. Also, if I don’t do my job, my income suffers. Additionally, there are other aspects of my life that need time and attention. While I’m very flattered that you might regard me so highly that you’d want my input, please don’t put me in the position of disappointing you by refusing your request. That would just make us both sad.
For the impolite people — hm. Well, you puzzle me. It’s hard for me to imagine how someone could just forward their stuff to a stranger without a even simple, “Hey, would you mind?” first.  But if you send me hard copy, I’ll either refuse delivery, or return it unopened, or shred it, unread; if you email it, I’ll delete it, also unread.
For a more extensive and much more entertaining treatment of the subject, check out John Scalzi’s blog post here. While I am nowhere near as busy or successful (or well-paid!) as Mr. Scalzi, most of his general principles apply in my case, too.
There used to be a great book on professional etiquette for writers, but alas, it’s out of print. Also, it’s out of date, being pre-internet and pre-email. But if you find a copy, the ideas are certainly still applicable.
That said, back to Book 5 — which still seems to require an inordinate amount of wrestling to get it to behave…