I used to own a six string all tung-oiled koa Carvin (yummy). I traded it long ago for an Iby EW series (quilt maple, yummy again). At any rate, I would say that Carvin necks are some of the best in the business. The quality of woods used is top shelf. If you dig on wizard necks, you'll love a Carvin neck (six string that is, the sevens are a bit fatter).
As far as tone goes, I always felt that my six lacked character. The Ibanez EW that came after it blew it away...but that was a mahogany guitar with PAFs. If you want to change pups in a Carvin, you'll need to do some routing as their pups are not standard width.
Build quality I found to be very very good. Mine was made to order and they got it done fast. But like Randy said, bang around in the used market first before dumping coin on a spanking new one. All things being equal, if I had the choice of a custom Carvin or a something akin to an Ibanez 3120...I'd take the Ibanez every time.