There's an old question that goes: If a tree falls in the woods and nobody is around, does it make a sound? In a similar vein, if you write the greatest novel in the history of the world but nobody knows about it, will you sell many books? It's possible, but not probable.
In essence, your job as a writer - especially an indie author - doesn't end once you upload that final, professionally-copyedited version with the too-cool-for-words cover art. You still have to get the word out about your book. That's not to say that some novels will not gain popularity and through great word-of-mouth, but I wouldn't put all my eggs in that basket as a virtual unknown. Basically, you will probably have to do some marketing (so build that into your budget).
In my own case, I've got several venues that I intend to use for marketing and promotion, but one that stood out to me was the prospect of a Goodreads advertising campaign. Goodreads bills itself as having "140 million pageviews and 19 million unique visitors a month." That's a lot of eyeballs, and I'm sure we'd all love to have them looking at ads for our indie novels. Thus, pulling the trigger here was really a no-brainer.
Goodreads' advertising system is a pay-per-click model. Thus, you pay every time someone clicks on your ad. The amount that you pay per click is based on how much you choose to bid, with the bid range being from $.10 to $300. (The default rate is $.50 per click.) You can fund fund an ad campaign with a certain amount of money and also place daily limits on how much is spent. For instance, you could place $10 in your account for a particular ad campaign, and then bid 10 cents per click with a $1 per day limit. At that rate, you would max out the account with 10 clicks per day over a ten-day period. However, Goodreads states that the average click-through rate (CTR) is .07%, meaning that to max out the daily limit your ad should expect to have about 14,285 views each day.
With respect to views, Goodreads uses a "complex algorithm" to determine which ads are shown on the site, and those with higher bids are given higher priority. In short, the higher your bid, the more your ad is displayed. That said, Goodreads states that the CTR is a greater determinant of which ads are shown more often - i.e., the more often your ad is clicked, the more often it is shown.
Personally, I've been bidding 30 cents per click and I'm pretty happy with the results. I have ad campaigns for both of my books, Sensation and Warden, and have set a daily limit of $5 for each. I've never come anywhere near to reaching my daily limit - even with two ads for each book - and I've had thousands of views for each campaign since starting my ad account on 5/6/13. (One thing I will say, however, is that once an ad gets at least one click I've noticed that the number of views increases significantly.)
Speaking of multiple ads, I have two for each book within each respective campaign; one is targeted to readers based on genre, while the other targets them based on authors. (This is in accordance with Goodreads' suggestions.) From my experience, those based on genre do exponentially better in terms of page views than those based on authors. Also, all ads in the same campaign are funded by the same pool of money, so you don't have to dump in a huge sum of money. regardless, your campaign will continue to run until you run out of moolah in your account.
So, in the grand scheme of things, is advertising on Goodreads worth it? I would say "Yes," but it also depends on what kind of value you feel you are getting out of the ads. For instance, if you are strongly adhering to a bean-counting philosophy, you may measure successful advertising solely in terms of resulting book sales. Thus, if your novel costs $2.99 and you're advertising it with a rate of 30 cents per click, then you may feel you need to sell at least one copy every 6 clicks or the ad is a failure. (Any more clicks-per-sale than that will result in a loss.)
From my perspective, I think it would be a mistake to measure success solely in terms of quantifiable data like sales. I believe you should also consider the amount of exposure you're getting. By way of example my novel Sensation has a completely abominable CTR of .01%. However, it has still had thousands of page views, resulting in it being added to several "to-read" lists on the site. (It also got a nice 5-star review.) And all of this has only cost me pennies thus far.
In short, I think advertising via Goodreads is an excellent option. Creating an ad is simple and easy; anyone interested can get started on Goodreads' advertising page. (A nice FAQ is also located here.) Also, two other people who do an excellent job of discussing this advertising program are Lindsay Buroker and Geoff Wakeling.