This just spoiled:
Sometimes — but not all the time — a Dark Confidant.
Better or worse than Bob?
My first impulse was that Pain Seer is worse, generally, than Dark Confidant. For example, presuming you have made your Pain Seer-including deck correctly, you don’t get a freebie on the first fresh turn you’ve got Pain Seer on the battlefield. I mean, unless the opponent has Blind Obedience down or some such.
So… Card down.
Beyond missing the extra card on the first / next turn, you have a general requirement with this card to attack. I mean of course there are other reasons why your Dark Confidant / Pain Seer / Dark Confidant wannabe might be tapped / ergo need to untap; you could be on the wrong side of an Icy Manipulator, or Master Decoy… But generally speaking, your creature will be tapped because you just swung.
So: You swing; presuming Pain Seer survives, you get to untap it and net a free card. If you don’t swing, you are far less likely to [have to] untap, and ergo won’t get the free card.
So generally speaking… Worse.
Worse than Dark Confidant, right?
Especially given the fact that you won’t generally get the first card, I’d put Pain Seer < Dark Confidant; but I don't think it is 100% cut-and-dried. Longtime listeners of the Top 8 Magic podcast (BTW BDM and I posted a new one just last week!) know that I think Dark Confidant is probably the most overrated creature of all time. I’ve always conceded that it is playable (fine… “quite good”) but just not the unconditional bee’s knees that everyone else seems to think. For example, I was once in the finals of a team PTQ where I instructed Paul Jordan to not attack with his Keiga, the Tide Star for lethal. We read the opponent for Shining Shoal (would have been lethal going the other way) and we could just watch the opponent die to his own Bob.
Moreover, I have been playing a lot of Modern lately. One thing I have noticed over and over is my opponents attempting to suicide their Dark Confidants into my various Elves and Merfolk. There’s a reason for that: You can’t very well control whether or not Bob kills you. His greatness comes at a cost… And that cost is sometimes your life.
Conversely — and a little perversely — you can leave your Pain Seer back (you know, the opposite of attempting to suicide your Dark Confidant) to avoid any chance of an accidental death.
Pain Seer isn’t objectively as strong as Dark Confidant… But to be fair Dark Confidant is widely considered one of the strongest two drops in the history of a game defined by great two drops. Given its inheritance of Dark Confidant’s attention to casting costs, you can’t just jam Pain Seer into any deck (limiting its efficacy somewhat). I do think that it might go in some sideboards for matchups where is it unlikely to be blocked (or where potential blockers are cards like Omenspeaker that will not prevent it from untapping and attacking to untap again).
Snap Judgment Rating: Role Player (likely upper mid-tier sideboard card)