Do you think Duck is genuinely hot for Peggy? Or do you think he's using her?

It occurred to me that when he couldn't get to Don by spiriting Peggy away, he then made the move to have an affair on her. Could he be using her for information in some vendetta against Don?

No I don't.  The whole thing really gave me the creeps.  It struck me as being very calculated and void of genuine emotion. 

I think the whole thing was a seduction from the first lunch he had scheduled with her and Pete.  There's nothing I've ever seen in Duck's character that leads me to conclude that he's completely self centered - think of his response in the boardroom when he found out that Don was beyond his control.

If Peggy were to leave and go to work at Grey, I think Duck would dump her in a minute if their relationship endangered his position.

Duck and Peggy. I nearly tossed my toast.

Duck doesn't love Peggy. He just wants her one way or the other. Last night, he got "the other."
Peggy is growing,but is still just a little bit naive. She still has that ingrained dictum to try to please.

I agree.  It was very creepy and made for a big facepalm for me... Oh, Peggy, why?

Yeah I think she's a little in over her head trying to navigate through the male-dominated arena without much guidance other than watching the weird interactions of the likes of Don and Pete.  The move was cringeworthy, but no doubt she will get around this predicament and eventually get what she wants.