For the record, I'm not against Self-Milling, I just don't think it works well in a Blue Mill deck. My Black Mill Deck uses it to the extreme. Gravepurge and Empty the Catacombs combined creatures with milling abilities make Self-Milling great, especially with Liliana of the Dark Realms providing all the mana I need to pull it off.
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This is true, on all accounts except one. When playing another Mil deck, having the Orb will certainly be a disadvantage, as they will certainly being milling you down as much you are them. Also in Mill on Mill games, there are fewer cards that will be untapping simply due to the amount of instants and sorceries. Therefore, in a pure Mill deck with no self-milling tactics, this card is more of a hinder than a help under these conditions.
I didn't know about Dream Twist. It is a wonderful add to this deck. Mesmeric Orb is a double-bladed sword however; it is also not legal in Modern.