It really depends on the sets. There is the problem of double dipping throughout, however.
The Borg set is pretty decent.
The Time Travel set is okay, although they double dip on Endgame, which was on the Borg set, and All Good Things, which is on the Q set.
The Q set contains every episode of Star Trek that ever featured Q. All other sets are fan-voted favourites, but this includes every Q episode. Therefore, it's a decent set for Q fans who haven't bought complete seasons.
The Klingon set is kind of meh. I question why Broken Bow, The Trouble With Tribbles, and Trials and Tribble-Ations were included since they aren't really Klingon centred episodes, they only feature Klingons. Plus Trials and Tribble-Ations was already on the Time Travel set. Enterprise, TOS, and DS9 had other Klingons centred episode that could have been used instead.
The Captain's Log set has good episode selection. Although it includes The City on the Edge of Forever, which was already on the Time Travel set. Voyager's Flashback is an odd choice considering it's a Tuvok centred episode, not a Janeway one. And then you have the really strange oddity of TATV being on it as a fan-voted episode. Scott Bakula even expresses his surprise that there are fans who liked the episode in his interview on the DVD.
The Alternate Realities set is kind of strange. First, when the voting was first announced, they promised all MU episodes would be included anyway. But DS9's The Emperor's New Cloak was left out. Then there are episodes that have nothing to do with alternate realities, like The Enemy Within, Turnabout Intruder, and Frame of Mind. And finally, the fact that the TOS episodes on this set are remastered versions of the episodes is something I don't approve of.