[I wrote this in a thread at Fred Clark's Slacktivist in response to people saying they hadn't read Left Behind outside of Fred's criticism, but it occurred to me that this might have use as a stand alone review.]
As someone who's read the books (just the main series, not the prequels, sequel, or side stories) I think what those who haven't are missing depends in large part on how you'd go about reading.
I first read the books flying through them at a pretty fast pace (for an idea of the pace, I think I read five or six in one weekend) and they were still ludicrously bad, but not nearly as horrifyingly bad as if you actually read them at a reasonable pace.
On the other hand, sometimes I've looked to see the context of what Fred quotes, and when I'm reading at a normal pace and actually paying attention rather than trying to get to the next vaguely interesting bit what stands out is that it is impossible for Fred to ever truly communicate how awful these books are without quoting everything. Everything he leaves out is a bit of terribleness you're spared and more than that the unrelenting badness works together in a way that somehow adds up to more than the some of its parts.
The books are more than just a bunch of wretched things, they're a vast expanse of never ending awful that you cannot understand unless you go there and slog through the wasteland and get a sense of how the endless trudge is never interrupted by even the shortest respite of good.
On the rare occasion that a good character somehow finds their way onto the page, they tend to be treated like utter crap. Often by God.
What you miss by not reading is the size and scope of the bad.
In the end, no one can be told how bad Left Behind is, you have to experience it for yourself.
Also, in my experience, the memory fades over time. It seems like every time I return to the books I'm surprised by how bad they manage to be. I know intellectually that they're pretty much indescribably bad, but that knowledge never seems to prepare me for the actual experience.