I have one. It works. Haven't had much opportunity to use it myself yet, other than in the pool when they left the heating off, but it's been noticeably warmer than skin
Main thing I've used is the hood, which has certainly kept my head warm every time.
Had a buddy once who wanted to stick with her 3mm hood as it was more comfortable but felt obliged to wear her 5mm as it was cold. Lent her my chillcheater hood & she wore it with her 3mm. Came back up amazed - she'd felt warmer than her 5mm would have kept her, she ordered her own hood the same day.
And didn't give me mine back until it arrived