I tried the search ads when it came out. Didn't work very well, so I stopped them after about 3 weeks. Many, many shops did the same thing.
I started them again this morning, with 3 popular listings and very specific keywords. Got views and favorites already, but no sales for now. I invested in the 5$ option. We'll see how this goes. I haven't got much hopes, though!
I saw in threads that some shops had very successful ads running, but most find the ads useless. Guess it depends on the kind of items you're selling, your photos (are they clickable enough?), the keywords you choose and the items...But if there's one tip I can tell you, definitely go with less items promoted and specific tags.