I've never used "concentrates" to make grape wines but I have used wine kits from Winexpert, Cru Select, and several others. The cheaper kits make "cheap wine", like $3-5 a bottle in quality, but the good kits make very good wines that are on par with commercial bottles in the $25 range.
I like kits that come with grape skins, as they seem to give the best results.
Broken Leg Brewery
Giving beer a leg to stand on since 2006