[This is going to be much more of an analytical post than a spiritual post. Please understand that I am not addressing my own spiritual witnesses or feelings in this post in any great detail.]
Could Joseph have written the Book of Mormon? (In other words, is it possible that it is nothing more than fiction?) Sure; to deny that possibility completely is silly, in my opinion. However, I personally don't view it that simply - for a number of reasons.
1) I really do believe it is a MUCH more complex record than many assume.
I think there are legitimate strengths and arguments to be made regarding: 1) chiasmus; 2) the geography of 1 Nephi (especially); 3) the cultural contrast between the Nephites, the Mulekites and the Jaredites (which, as a history teacher, I can tell you are striking and incredibly accurate for what we know of the groups that might have been the genesis of each group); 4) the consistency of the split-narrative format; 5) the various narrative voices and styles (that shift quite seamlessly, especially when Mormon is recorded as inserting his own commentary smack dab in the middle of quotes from other sources); and more. I don't pretend that these things "prove" Joseph wasn't the author, but there are much stronger arguments than most people (even members) realize.
2) Joseph really didn't understand the Book of Mormon very well - and very few authors misunderstand what their own works actually say to the degree that Joseph did. His lack of understanding is a pretty good argument for the manner in which the transmission is said to have occurred.
3) There is an element of fluidity in the descriptions of the translation process that would be difficult for someone who simply was making it up - or who had done nothing more than map out a story in his mind over time.
4) In order for it to have been a fraud, as it would have to be if Joseph wasn't sincere in his description of what happened, there would have had to have been a fairly wide-spread conspiracy - and I just don't see that in the records we have available to us, even from anti-Mormon sources of the time.
5) I'm not really concerned all that much about what others perceive to be anachronisms - especially since the record itself does a pretty good job explaining why the doctrinal ones are there. As to nearly all of the others, the question of elephants and steel (which were the focus of much of the early criticism of the Book of Mormon) is perhaps the best example of why I don't spend emotional or intellectual capital worrying about them. Since the time the Book of Mormon was published, LOTS of stories of mastodons roaming the American continent(s) have been discovered in the oral traditions of the American Indians that would fit the reference in the Book of Ether (from dictionary.com, for example: "a massive, elephant-like mammal that flourished worldwide from the Miocene through the Pleistocene epochs and, in North America, into recent times), and "steel" is much like "man" - a very generic term that could mean much more than the most narrow parsing of the word itself.
6) All of the books that are listed as inspiration for Joseph to have written the Book of Mormon actually are RADICALLY different than it. The central themes, the narrative flow and voices, the cultural statements - pretty much everything indicates that the Book of Mormon isn't derived from those sources. A non-believer could argue reasonably that the genesis of the idea to write the Book of Mormon came from the existence of such books, but I find the idea that they actually provided source texts to be quite weak.
I personally think there is a better argument for "delusion" than for "deceit" - but I choose to believe it actually was an inspired transmission. That is a conscious choice, and I understand very well the arguments against it, but it's what I have chosen to believe on faith. I've read it slowly and carefully and analytically and parsing as I go - and I honestly believe most members still have little idea what it actually contains within its pages.