I've bought those $1 ads for two days in the row some time ago. In those days the count of visitors per day was more, than usual, but less than at day when I've reach popular section on tapas (even despite I lasted here just for a couple of hours, while ads are active for all day). And reaching this section is free, it just requires active readers and luck to get enough involvement from them after releasing new episode + (desirable) new sub in the short interval of time at once. Also I didn't get new subs with those ads.
So, if you have enough money, it will not hurt to buy those ads. But if you are not rich, maybe it will be better to spend them on something more useful.