I kinda thought the Delta Species set was a letdown.
Yes, I like the fact that you now have Pokemon who's DNA seems to dramatically changed that they're now becoming types that they previously never have(ie. Holon Phantoms will feature an Electric type Charizard). And, making Electrode and Magneton energy cards is an interesting effect too. But, the playability of some of these cards, at least in the current DeltaSpecies set, is limited by the quality of the moves/pokepowers provided. Yes, having an Electric/Steel based Metagross is a nice change to an electric deck, but the moveset on Metagross isn't that great.
My feeling is this - If you provide these cross-types with a strong moveset and HP, you'll make many of the previous cards released in other sets obsolete. If, however, you continue to issue Delta Pokemon cards with rather mediocre attacks, minimal HP, etc., then you'll have more sets that go the way of the Crystal Pokemon types and the Pokemon VS sets(JPN released only).