I consider depth of field preview essential. I will not buy an SLR that doesn't have it. The only reason I have one that doesn't is that it was given to me.
I don't use it all the time of course, but I use it a LOT. You get used to it. Especially in a camera with a nice bright viewfinder like my LX and MX, I consider it extremely useful.
Of course I'm used to sticking my head under a darkcloth and making the same evaluation on a view camera ground glass at often smaller apertures. Maybe it takes practice. I've been doing it so long I'm not sure.