First up, these aren't as good as the cards they represent. Mainly because a lot of these have tap abilities that can't be used on the first turn like the originals, but also because creatures are more vulnerable than artifacts/enchantments/lands. Second, due to the large size and repeated general comments, I'm just going to make a quick list of pros and cons and mention which cards they apply to.
Pro: Good power/toughness for cost. These cards are good enough as creatures for their costs.
Cards: Abyss, Arena, Candelabra, Coffers, Moon, Scroll, Vineyard
Pro: Costs the same as the original. These cards, while usually being slower than the cards they represent, at least cost the same as the original.
Cards: Abyss, Candelabra, Disk, Future, Jar (sort of), Mirror (sort of), Moon, Scroll, Vineyard
Con: Overcosted. These cards cost more than their originals, sometimes a lot more.
Cards: Arena, Bazaar, Coffers, Library, Tabernacle
Con: It can be affected by its own effect. These cards can be destroyed by using their own effects.
Cards: Abyss, Arena, Tabernacle
Con: Weak. These cards don't have a good power/toughness ratio for their cost.
Cards: Bazaar, Disk, Future, Jar, Library, Mirror, Moat, Tabernacle
Cards: Abyss, Candelabra, Moat, Moon, Scroll
Cards: Arena, Coffers, Disk, Future, Tabernacle, Vineyard
Cards: Bazaar, Jar, Library, Mirror