I hired a car in Las Vegas back in November.

To be honest, driving up the Strip isn't all it's cracked up to be - loads of traffic, red lights every few metres etc so you end up stop/start all the time and it takes ages.
As for parking, saw some at the Bellagio and Mandaly Bay but some casinos would have you drivng around the streets trying to find where to park and with heavy traffic, you can easily miss the turn.
It can be a bit of a walk between say the "Welcome To Las Vegas" sign and the Venetian so driving may be better there. When walking, it's a pain stopping at every crosswalk, having to go up escalators, crossing the street multiple times as the sidewalk vanishes, fighting your way through hundreds of people and touts / people handing out flyers etc.

Far better to have one if you want to explore off the Strip. I drove to Hoover Dam, Boulder, Jean, Primm, Goodpsrings and some other places which made a refreshing change from casinos that are boring after a few minutes (unless you can afford to throw away $100s or $1000s all day)

That said, car hire can be cheap and having one is useful for popping out somewhere or getting to and from the airport that would cost you up to $20 each way anyway (unless you stay somewhere with a shuttle bus and you generally have to call the hotel to get them to pick you up anyway which can take half an hour)