The Mob Museum is a good way to draw your interest, but the museum is actually a larger selection including the history of Las Vegas and the mobster history combined into one large and interesting place. The museum is in the former federal courthouse in downtown Las Vegas, just two blocks north of the Fremont Street Experience.
Admission is rather steep, but the collection is rather extensive with lots of unique and interactive exhibits. I felt the museum was actually a bit light on the history of organized crime, but they made up for this by being an unofficial Las Vegas history museum with lots of interesting history. You start at the top and work your way down with several movies and multimedia displays. The subject matter is rather dark and violent, so I do not recommend bringing children.
This is an enjoyable museum experience for everyone, even though the subject matter depicts the darker side of human nature. Most people were fascinated with the exhibits, although self selection probably ensured this. The museum is a little chaotic with the progression of subject matter, likely still finding its rhythm. The architecture of the original courthouse has been preserved making it even more interesting to wander the halls.
I really enjoyed my trip to the Mob Museum. They have combination tickets with the Neon Museum and other activities which are also recommended. It does take a long time to work through the exhibits, but they are open later than most museums to help.