From my inexperienced point of view this looks like a good deal for these slides. I would be using 18":
I realize there are a LOT of variations on these slides. I would use them in a buffet / sideboard for my own use. This piece would hold table settings, table cloths, runners in a center group of drawers. Serving dishes and larger items in the outer / larger drawers. This is absolutely NOT the final version but, to give you the idea. Approx. 72" x 21" x about 36" tall.
Sideboard Idea-ver 11.jpg
I absolutely don't want to cheap-out but, these are coming in about half the price of the ones I had been looking at. Also Blum but, full extension.
Friends, Cabinet-makers, Countrymen, . . . am I missing something? Tell me all about it . . .
Extra Credit follow up question. Should I spring for a drilling jig or can a hack like me fumble through it for this and (hopefully) many future drawers?