I think Immortal throne adds enough features and fixes to warrant $15 (caravan storage, transfer items between characters, fixes to some game play issues,etc). I have played through Normal once and almost thorugh Epic with one character. I have a few characters in different stages of normal as well.
The items and sets make this game really neat. Although I only seem to find a crap load of Santa set items anymore.
Multiplayer is pretty good, but finding a server without cheaters is sometimes a challenge.
Vista requires running this as an admin for network play. I know that people had issues with the steam version, but if Impulse has an executable, try running as Administrator to get network play to function.
FWIW, I am playing quite successfully on a Vista ultimate x64 with few issues (random crashes to desktop is about it).
EH