I just got in S5 from the DD sale, and watched the first ep the other night.
I still have season 4 in the shrinkwrap so I don't know what the PQ of that one is like, but I immediately noticed how much poorer S5 looks in relation to the first three seasons.
If I had to guess, I would say that the first three (four?) seasons were remastered, and for Season 5 (and beyond?) they just used whatever existing masters were available. I'm seeing a very soft, lower res image replete with the usual moire effects you get with them. In addition, compression doesn't look like it was attended to very well. Very disappointed how much Fox seems to have lowballed this property.
I'm sure we'll get the remaining seasons without too much further waiting, but I'm also sure it will not be with anywhere near the care they put forth in the past.
That's a real shame.
I'll still pick the rest up, but I will definitely not be rushing out to get them. I'll wait until there is an 'offer I can't refuse' sale.
I really wish I'd waited for that on this one