Only did NYE at Niagara once and not likely again. It wasn't super cold, but it still is chilly standing around outside for hours. I think the weather forecast is pretty damned cold for NYE 2013. You might not see the show too depending on where you stand. They have video screens and all, but you're basically looking at people's heads for hours. The ground gets pretty muddy and slippery with thousands of people standing around. Stay away from Clifton Hill afterwards because that's where everyone goes after the show for coffee / hot chocolate. Then there's the trek back to your hotel or car, which is a pain.
If you're a young buck and eager to party than go the fuck out.
I on the other hand will stay home in peaceful bliss drinking the premium wine and beer that I choose in peaceful bliss.