On Dec. 27, as Britney Spears kicked off her much-hyped "Piece of Me" residency at Las Vegas's Planet Hollywood resort, descending from the vaulted ceiling in a futuristic gilded cage while wearing what appeared to be her rhinestoned nude bodysuit from the 2000 VMAs, the burning question inside the 4,600-seat theater was: Was she singing live?

The answer: Eh, probably not. Not most of the time, anyway. The relatively intimate setting made that pretty obvious. But then, there was another, almost equally valid question: Did it even matter? And once again, the answer was, well, no. Not really.

Real-time reactions inside the theater to Britney's 90-minute Vegas revue — which included her flying through the air dressed as a snowy Christmas angel for "Everytime"; a reenactment of the Cardio Barre choreo from her "3" video; a totally-'90s, ravey "Scream & Shout" duet with a virtual, Max Headroom-like will.i.am; a rainforest performance of "Toxic" (toxic rain?) during which Britney scaled a giant plastic tree; and several dance routines seemingly inspired by Paula Abdul's "Cold-Hearted" video — were mixed. Some spectators seemed peeved by Britney's apparent lack of actual singing, tweeting their snarky disdain:

Other, more supportive audience members championed Britney, showing little interest in any ongoing live-or-Memorex debate and expressing giddy delight that the pop princess, whose famously fancy footwork never was quite as fancy after her 2004 knee injury, was enthusiastically dancing again:

And then there was Britney's number-one fan, occasional duet partner, and possible successor, the one and only Miley Cyrus, twerking up a storm in the front row. Miley certainly seemed thrilled with the show:

Let's face it, Britney Spears lip-synching, or singing to track, is hardly a scandal of Milli Vanilli proportions. After all, this was not a Celine Dion-at-Caesar's situation. Vegas tourists buy Celine tickets to witness world-class singing; they buy Britney tickets to be entertained. And in the entertainment department, Britney mostly delivered. She changed her costume seven times (and her wig twice!), she hoofed in high-heeled sneakers, she looked fabulous, and she ran through all of her most crowd-rousing hits. She did work, b----.

Miley wasn't the only appreciative celebrity spectator at Britney's opening night. Katy Perry, Selena Gomez, Adam Lambert, Sia, and Pauly D were all in attendance, and Britney's ex-"X Factor" co-star, Mario Lopez, even got in on the action, when Britney looked for a "hottie" in the crowd, then dragged Mario up onstage for a mildly titillating S&M-themed vignette, during which she wrapped the former A.C. Slater in chains and swatted him with a riding crop. (Important side note: This was as sexy as the 32-year-old mother-of-two ever got onstage. Vegas visitors in search of a truly racy revue should probably buy tickets for Crazy Girls instead.)

Time will tell if Britney's Planet Hollywood residency (which will involve her playing 96 shows over the next two years) is a career risk that pays off. With her new Britney Jean album a relative sales disappointment and her aforementioned one-season "X Factor" stint a bust, a Vegas showcase (especially one featuring less-than-stellar singing) could be seen as a desperate or downward move. And truth be told, one between-song jumbotron mashup of Brit Brit's greatest music video moments did sort of drive home the point that her glory days may be behind her. But as Britney ended her first Planet Hollywood show, she seemed happy and in surprising command of the stage, smiling and squealing, "I feel high!" before joining her troupe of 14 dancers for a jubilant "Till the World Ends." And after all her personal and professional troubles, just seeing Britney enjoying herself onstage again was, well, something to sing about.

Britney's full "Piece of Me" setlist was as follows:

Work B----
Womanizer
3
Everytime
...Baby One More Time
Oops…I Did It Again
Me Against the Music
Gimme More
Break the Ice
Piece of Me
Scream & Shout
Boys
Perfume
I'm a Slave 4 U
Freakshow
Do Somethin'
Circus
I Wanna Go
Lucky
Toxic
Stronger
(You Drive Me) Crazy
Till the World Ends

Related links:

• The song Britney is sick of singing

• Britney's dad wins duct tape jackpot