Solid. F18s are over-built, so I have never heard of issues like delamination or hull failure due to a missing bulkhead or something. There are a ton of top-shelf F18s for sale at stunningly low prices right now due to the introduction of the Capricorn, the Infusion, and now the 2007 Tiger. You can pretty much take your pick of designs. The only issue on a three-year-old boat might be the age of rigging and sails (particularly the jib and spin). It is a buyers market if you are willing to go with a used Nacra or Tiger, so you are likely to find a great, well-maintained, tricked-out race boat with sails that are only a few regattas old.
I would caution against only one brand - Mystere. You might find a great deal, but the supply of parts from the manufacturer has gotten iffy.
There are even used Caps and Infusions on the market now! <img src="http://www.catsailor.com/forums/images/graemlins/wink.gif" alt="" />
Have fun shopping. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />