Funny enough, I can't stand using the IR scope on Hanoi. The background just looks like fuzz to me, and I can never tell if I'm looking where I intended unless there's a target standing there. However, I love it on Discovery and other well-lit maps.
For the longest time, I ran the PSG-1 with the IR Scope. It didn't matter the map. It didn't matter whether the opponents were wearing Ghost Pro(Note: If you are wearing Ghost Pro, you will not be highlighted by IR Scopes if you did not already know). I would snipe like crazy. I just got bored with it. I now use the Dragunov or the L96A1 these days. I usually equip the Dragunov with a Variable Zoom and the L96 with Extended Magazines.
As far as the IR Scope goes. If you like it, go with it. It can work just fine on any map. It does take a little getting used to. But all in all, it is just as solid as any other attatchment in the right hands.