As a card, though, Purity is okay. Being a 6/6 flyer for 6 mana is certainly playable, but requiring 3 white as part of its casting cost limits its play-ability a bit, except in Commander where mana-fixing is much less of a problem that in a regular 60-card deck Constructed format.
Its abilities are kind of cool. As with the other Incarnations in Lorwyn, such as Guile (which is another one of my favorite Lorwyn creatures), whenever it would be put into a graveyard, it is shuffled back into its owner's deck instead. This includes being discarded from the top of the deck, as well, so mill strategies will be unable to finish the job with this card in your deck.
Also, Purity's other ability prevents you from spells or abilities dealing damage to you, allowing you to gain that much life instead. However, remember that damage is not the same as loss of life. Spells and Abilities that cause you to lose life but don't explicitly say damage are not helped by Purity's ability.
Purity is a bit of a situational card, best used in Commander play-groups where direct damage dealing is common. Otherwise, it's just a flavorful card that is outclassed by a lot of other things that would take its card slot in your deck.