I'm thinking in particular with the KDP platform rather than any other distribution method, but any general tips welcome. It seems like a great way to build a word-of-mouth buzz and to get some positive reviews. I imagine that people are more willing to give a positive review for something they got for free than something they had to pay for (less chance of feeling ripped off).
However, I'm also aware that it's hard to make money giving stuff away, even if it is only a digital product. Do you guys feel that the sales lost by promoting the book as free is outweighed by the sales gained from the extra exposure?
Intuitively to me it seems like it should, because if you want to make a living in this game you need to shift a lot of copies, and if you have no name (like me) you need to get one before you die, and promoting the book as free seems like a good way to do this.
Some randomish thoughts about free promos in no particular order....
The only place free seems to have an impact on future sales is Amazon (for items outside of series novels.) Amazon's discovery algorithms and traffic seem to be leaps and bounds better. Before/after free is identical for me on every other retailer.
If you aren't selling really well out of the gate, you have nothing to lose by trying a short free period at Amazon. Sales can't get much worse.

I don't think the "lost sales" argument holds much water. The battle is with obscurity, not with missing a couple bucks here or there.
Lots of free downloads appear to be collectors. It's unlikely that the majority will even read what they downloaded. (ie. keep your expectations in check for what a free download actually means.)
Going free multiple times seems to suffer from diminishing returns. Seed the Amazon system through a free promo once, and be done with it until sales drop to near zero at which point you're at the "nothing to lose" stage again.
Reviews seem to come at a pace of about 1000 to 1. If you're looking to get reviews out of a free promo, you need to give away a LOT.
Don't assume reviews will be more lenient because of the price. Many free hoarders will download books out of their preferred reading genres and will ding you even if their preferences don't tend to match your target audience. Some authors advise waiting until you have a few good reviews before trying free just in case you lose review lotto and get dinged with a 1 or 2 star out of it. (As rare as reviews are for a newbie, I'd gamble, but its seems worth sharing the alternate viewpoint and the truth it is based on.)
Free tends to work better when you have a few very similar things to sell in addition to the freebie. Series best of all, other books in very similar genre, a free short story related to a book, etc.
On Amazon, one of my free short-fiction promos caught 1000+ downloads, a 5 star review and went from selling about a copy a month to close to a copy a day. It's still selling a copy or two a week a few months later. So lunch money, but in the realm of short fiction where typical results are 0-5 copies a month it's doing reasonably well thanks to that free promotion. I was unable to duplicate that level of success with another story even on Amazon. I'm now in the middle of trying it with a third but may not hit the 1000+ download goal before it goes back to paid.