Let's start by my saying that I think this is the best-played hand by Jim all night.
As I was thinking about the call ($68 to me, about $105 in the pot already) I looked back at the board and thought about what he could have. Board was Ah-6d-7d.
Best Case: A-J, A-10, A-9
Worst Case: 6-6, 7-7, A-7, 6-7 suited
Most Likely - Ad-Xd, Kd-Xd, 8x-9x, 4x-5x, two random diamonds
Least Likely - Ace-rag, A-6 (one of these is very bad, one of these is very good for me, but both are exceedingly unlikely)
So I thought about the hands he could have and figured that I was probably ahead of 60% of his possible holdings. Note that I never for a minute put him on Aces or A-K, or any pocket pair 9-9 or higher for that matter. If Jim had any of those holdings he wouldn't have limped in late position in a straddled hand with a million other people limping already, so I figured it was about 99% unlikely that he had any of those hands.
So since I figured I was ahead of around 2/3 of the hands he could possibly hold, I made the call.
He tabled 8d-9d for the open-ended straight flush draw, and I was drawing dead after the 5s on the turn. The 5d on the river was just the icing on the cake.
I really do think this was an excellent play on both our parts. As Special K said, "you got your money in ahead. You weren't the favorite, but you were ahead." And if I'm not going to be willing to put my money in ahead, I shouldn't be playing. Jim made the best possible move, ensuring that he was going to get the greatest possible value for his hand, because if he just check-calls the flop there, I'm done if another diamond hits, and might not invest much more if a 5 or 10 hits. I'm not sure I have that flop bet in my arsenal, but it's a good move.
I still think it wouldn't have been a huge mistake for me to make the laydown there, but I made a read, stuck with it, and was correct. The next two cards sucked out loud, but that's poker.