I think the general consensus is to just avoid them. I know a few people have tried them, and only find one silver every 4 boxes or so. They could be fun, but I'd stay away.
Of all the denominations, the only two I don't do regularly are small dollars (very rare) and quarters (somewhat rare).
Silver quarters are still able to be found, but other denominations will usually lead to more success.
Here are my quarters numbers for the past two years
In 2007:
13 silver quarters out of 105,799 quarters searched....which works out to a little over $2034 searched for each silver quarter....or 1 for every 4 boxes or so.
In 2008:
5 silver quarter (+2 Canadian silver) out of 47,468 quarters searched....which works out to a around $1695 for each silver quarter (US + Canadian)....or one every 3.5 boxes or so. (every 4.5 boxes if just US silver were counted).
So combined for the two years...roughly 76 boxes worth searched for 20 silver coins (18 U.S.).....
Worth it....depends on how much you enjoy the thrill of the hunt, or if you are just after silver....
I'm at 2 for 6 boxes. two boxes had one apiece, both came from the same branch (other boxes from diff branches) so when I go there to dump halves I will try another box till the well runs dry. HH Mark