In Eldrazi Commander decks, you get to copy your more powerful Eldrazi or other colorless creatures for 5 mana at the cost of having to sacrifice Mirrorpool. This is usually well worth it. Copying Wurmcoil Engine or Myr Battlesphere feels pretty good, as does Blightsteel Colossus. Endbringer and Deceiver of Form are a couple other Eldrazi worth copying with Mirrorpool’s creature-copying effect.
As far as copying an instant or sorcery for 3 mana, there aren’t a ton of options. Copying Titan’s Presence to exile two creatures instead of one is pretty good, though. Skittering Invasion is the other good one to copy, making another 5 0/1 Eldrazi Spawn tokens. That seems worth it. Scour from Existence is another excellent one, allowing you to exile two permanents for 10 mana and a land. That doesn’t actually sound excellent, but it depends on how much you need to exile that second permanent. You’re still actually not down a card either way.
The Gitrog Monster is a deck all about lands. Not only are there plenty of good creatures, instants, and sorceries in that deck to copy, but when you sacrifice it with the “Hypno Toad” on the battlefield, you get to draw a card. Not only that, Gitrog Monster decks have plenty of ways to get back lands, such as Crucible of Worlds and Life from the Loam.
Honestly, it doesn’t seem like Mirrorpool sees nearly enough play considering what it’s capable of doing. Being a colorless only land, it seems like it’s being overlooked more than it should be.