Marline asks: Kindle Countdown yea or nay?

Hi, gang! I’ve been ridiculously busy promoting my debut novel AND my debut novella over the last two months and now, I’m hoping for feedback from those of you who’ve used Kindle’s Countdown promo feature (for those with KDP Select). Can I get a witness on how it worked for you, what you wish you’d done (or what wasn’t worth the effort), timing, tips, etc. Thanks in advance!!