I found it very amusing to see how people's draft stock would rise the day after a good preseason game, even though they seem to have little to no correlation with the regular season. Obviously Aaron Rodgers has been going pretty high all month long, but after the Packers put up 59 points, I saw him going several slots higher on average in the next several drafts; sometimes more. Not that I'm complaining.
I think the only real value in the preseason lies in the extremes: if someone looks terrible, it might mean something, particularly if they're older, and if someone looks great, you can at least consider the possibility that they'll get some playing time (or more than you'd expected). Small, meaningless sample though it is, I'm sure coaches base decisions on it, anyway.
Other than that, though, it's just about monitoring injuries and hoping other people put too much stock in it.
I think the only real value in the preseason lies in the extremes: if someone looks terrible, it might mean something, particularly if they're older, and if someone looks great, you can at least consider the possibility that they'll get some playing time (or more than you'd expected). Small, meaningless sample though it is, I'm sure coaches base decisions on it, anyway.
Other than that, though, it's just about monitoring injuries and hoping other people put too much stock in it.