I have mixed opinions about them, but I think it can be a good idea to use them when you have some excellent content you want to promote. I think my understanding of them is not the best though, I have seen some people who %%# post three times per day and bid up each one massively. Any idea why they do this? It doesn't look like they are making a profit
It's possible some of the larger votes on bots with higher min bids allowed may have a higher return as less are using them due to the cost. I have no clue if thats true of not as I don't have enough SBD to test that out or haven't done any research on it.
We also have to keep in mind that we not only get SBD returned on our bids but also Steem Power. So we have to calculate the value of both we receive and compare it to our bid to get a more accurate ROI percentage.
If we break even or slightly under with out bid we gained allot more exposure on our promoted post then if we didn't which hopefully gave us other benefits such as more followers that will upvote our future post.
Think of it this way .... in most instances when you spend money to promote a service / product there is no gntd of any return. Here on Steemit ( with a little research ) you are almost at least gntd to get 90 % + of that money back.