Well, depending on which site, you get either 3 or 5 ghouls, and while I wouldn't do it while still researching, I might do it after finishing research, if a death mage happened to be there already.
As for which "enter to summon" units are worth it, I'd submit that the following are usually worth it, and the rest are situationally worth it.
Code:
Devil's Den 304 Devil 1
Dragon Jaws 593 Draconian 1
Dragon Mountain 593 Draconian 1
Jungle Temple 394 Lamia 1
Kraken Pit 438 Kraken 1
Mount Frost 511 Winter Wolf 2
Skull Temple 535 Longdead Archer 1, 189 Longdead Horseman 1
Temple of Darkness 675 Shadow 1, 676 Shade 1
The Bowl of the Lost 185 Bane 1
The Demon Gate 88 Fiend of Darkness 1
The Factory 532 Mechanical Man 1
The Forest Beneath the Waves 1062 Sea Stag 1
The Hippocampoi Fields 1058 Hippocampus 1
The Lesser Abyss 88 Fiend of Darkness 1
The Mountain of Power 403 Horned Serpent 1
The Sea of Dead Memories 676 Shade 1
The Shaded Lands 676 Shade 1
Tomb of Seven Oracles 1501 Cavern Wight 1
Tower of Seven Tombs 625 Mummy 1
Valley of the Dead 625 Mummy 1