For raid-utility Affliction right now, I'm quite happy with the Imp. He stays out of the way (Phase Shift), and is squeezable for mana on par with a Super Mana Potion every minute or so. He also provides Blood Pact to the group, which isn't a *lot* of extra HP, but it does help. Overall though, the main reason I keep him around is for mana.
For me to swap to the Felpup, he'd have to provide *better* mana than the Imp. This either means a much larger pool (the Imp is tapped out in ~1.5 Dark Pacts), better mana regen (which it looks like this talent is for), or both. Preferably both, as he'll require more management (careful about positioning, aggro, etc...), and won't provide the HP benefit the Imp does. Plus, it requires talent points...which the Imp technically doesn't.
I should really play around with this a bit in the beta, if the lag ever calms down enough to be bearable.