it's not that the dual cores are slower as a whole, but simply that there are less apps that can fully take advantage of it. I mean, using a skyrocket, theres like, slightly less delay flipping through a million menus, but on a day to day basis, its no problem.
expect a beautiful screen, and snappy response after you install one of the custom roms. If you're planning on using it as a music player, supercurio said it has the best audio chip/codec on the market right now. And avoid the samsung based roms if you do, they don't like reading ID3 tags properly.
as a note, i recommend CM7 with the thunderbolt script installed. fking amazing.
nothing really comes to mind for cons, i mean, it's fast, but not the fastest. is it fast enough? i'd say so. screens amazing as i already said, battery lasts me a good day with heavy usage. HD videos don't lag, i can play PSX games on it no problem, overall pretty solid