Hi Randy,
See my report at http://www.findmall.com/cgi-bin/forums/mxt_detector/whites.pl?read=6916
Bottom line is I think the 18" is a good coil, but it is certainly a niche coil due to its size and weight. It is not for everyone, but for someone like me it is a "must-have".
I think one reason I do well compared to some people I have hunted with is I have multiple machines with multiple coils. I personally have no use at all for the "best machine" mentality as I do not believe there is any such thing. So when I hit a certain area with a another person 90% of the time they only have one machine and one coil that they use for everything. I'll have a particular unit/coil combo I picked just for that days detecting out of my detecting "tool box". Having that ability to customize my tools for the job does give me that little edge over the other guy.
So that 18" will be the coil for that particular set of circumstances I'll run into someday. The other guy will be saying something like "well, yeah, you found more stuff. You covered more ground with that big coil than I could" or "well, yeah, you found more stuff. You got more depth with that big coil than I could".
A good set of wrenches will beat a pair of pliers in most cases.
Steve Herschbach