I tried it with one of my books when it first came out, but didn't have much success. The main boost you get from KDP Select comes with the free promotion days (it didn't get any lends for the entire three months), but I had less than 400 downloads, and didn't gain much sales momentum from it.
I probably didn't do enough promotion for it though, not having been on twitter at the time, and the best place to advertise free promotions is on Pixels of Ink, which, despite me submitting in plenty of time, was busy that weekend of my second promotion so I didn't get a mention.
Other writers I know have had better luck. One had 13,800 downloads from a three day promotion last weekend, and sales continued strongly when it went back to priced, having 2-3 months worth of average sales in a couple of days (she'd also dropped the price to $0.99 coming out of the free promotion). Here's her blog entry on what she did -
http://www.goodreads.com/author_blog_posts/3057534-kdp-free-promotions---a-few-things-i-have-learned.
So if you're good with social media (I'm not), it could give you a useful boost to your exposure. One of the main concerns is that the people downloading free stuff may never get around to reading all of them. Even if they don't, the more downloads you get the higher up you go in the rankings, so the more chance you have of being found by someone who will enjoy your work (this is why multiple day promotions are better, to keep up momentum to reach higher ranks).