I went to Vegas for New Years a few years back, it was pretty fun, but it is expensive! VEGAS IS PACKED! so many people, good luck getting a table under $10 (unless you go off the strip of course).
The shows are all going. Although, i really dont remember christmas decorations to be honest...
New Years Eve is a ton of fun. They close down the strip so cars cannot drive on the strip, and its just a major party outside. The weather was cold (i'm from CA so i thought it was cold). I'd say 40's was the low. But, lets be honest, usually you have some drinks in you in Vegas so its really not that cold out
. Its funny, the strip is closed, people are drunk, girls are showing stuff, and the "Church" comes out with signs telling people how they are sinners.
Like i said, its expensive, you pay the premium for being at one of the best new years partys. Everything is packed though, you go the week after and you will find 5 star hotels for $70, table minimums for $5, and atractions will be empty (thats when i go now that i'm an old man at 32)
If you dont like crowds, i wouldnt go to Vegas for new years, if you enjoy the people, madness, etc., you will love Vegas on new years