My "Britney: Piece of Me" experience was absolutely amazing. You basically get to drink delicious cocktails while singing and dancing along to your favorite songs from the 90s/early 2000s.
As many other reviewers have pointed out, she does not sing live. I can certainly understand why that bothers people. However, it did not bother me. Britney's dancing and overall stage presence made up for the fact that we were singing along to a pre-recorded track. I am beyond impressed that she is able to put on a show like that every night.
She performs all of her big hits (I hate when artists try to push their new stuff and ignore the songs that made them popular). The costumes and props/scenery are very Vegas- lots of lasers, fireworks and beautiful sets. The choreography is exceptional. The volume is perfect. She really knew how to hype up the audience. Everyone was on their feet dancing. She seems genuinely happy to be performing for her fans. I loved it.
My only complaint is that I wish it was a longer show, however, I understand that it would be difficult to do a 2-3 hour concert every night.