Are Kindle Countdown Sales Worthwhile?

Last Thursday, I ran a Kindle Countdown Sale to promote Bluewater Killer, the first book in my Bluewater Thriller series.  Besides scheduling my own Tweets and requesting Twitter support from fellow writers, I sponsored Kindle Nation Daily's Kindle Daily Deal newsletter.  Kindle Nation Daily featured my book at the top of their mailing and their web page for the day.  
The book's regular price is $2.99. On the first day, when the book was priced at 99¢, I sold thirteen times my average daily unit volume for that title.   That was the day that the paid ad ran.  The book was ranked at about 65,000 in the Kindle Store the morning before the promotion.  Over the last few months, it has frequently been ranked in the 30,000 to 40,000 range.  Bluewater Killer has often been in the top 100 on the Action /  Adventure / Sea Adventure list.  On the first day of the promotion, the ranking peaked at about 4,000 in the Kindle Store, and it reached number 7 on the Sea Adventure list.  
The second day, when the price was $1.99, the only promotion was on Twitter.  Sales for the second day were minuscule -- one half of the long term average daily rate, and the book began a slow decline in ratings.  As of Sunday, it's ranked around 25,000 in the Kindle Store and around 50 on the Sea Adventures list.  Daily sales now are about average.   
The increase in total royalties for the first day offset a substantial portion of the cost of the paid advertisement.  Because I haven't used Kindle Nation Daily before, at least some of the purchasers are probably new to my work.  I also think that someone who buys a book, even for 99¢, is more likely to read it than someone who picked it up for free.  If 10 percent of the people who bought Bluewater Killer for 99¢ go on to read the rest of the books in the series, I'll more than recover the cost of the ad.  I didn't see any significant change in the sales of my other titles during this promotion.
I don't think that the countdown sale provided any benefit over previous straight 99¢ sales that I've done, but I did see a dramatic boost from the paid advertisement.  Without paid advertising, 99¢ sales haven't increased unit quantities enough to offset my lost royalties from the lower price.  In this case total royalties for the title were slightly higher than my daily average.  With no loss in royalties during the sale, my only expense was for the ad.  I think the paid promotion was worth doing.  It didn't change my short term income, but it probably expanded my base of readers a bit.  I'll continue to use paid advertising as part of my marketing mix in the future, but I'm leaning toward heavily promoted, single day 99¢ sales as opposed to countdown sales.
I hope this is useful to other independent authors.  Comments, questions, and suggestions are welcome.  What's your opinion, either as a writer or a book buyer?