At this writing, I've been to The Venetian twice, once as a hotel guest for a conference and once for dinner, a show, and a late night bite.
As a hotel guest, I appreciated the friendly staff and the size of my room, but was disappointed by the cigarette burns on the sofa (in a non-smoking room, I might add) and some careless housekeeping. It was nice to have so much to do under one roof but boy was is a schlep from my room the the conference center and other hotel amenities!
Although the hotel and casino probably isn't super smoky to most people, I was bothered by it both times, especially when I was in the hotel for dinner, a show, and a late night bite, perhaps because I couldn't escape to a smoke-free zone.
I don't have a lot of experience with Las Vega$ but this is a hotel I would return to.