My experience has been that the return on investment isn't huge, but you generally do get back more than you put in if you know which bots are the best to use. For example, Hive account@lovejuice is a relatively poorly utilised bot with a good payout percentage.

You need to check the bots Steem power before using them. Low voting power means poor return. High Steem power means your return should be good. Of course, there's no guarantee so you're playing with fire sometimes. Don't expect huge profits though. It's really just for getting improved exposure.

But as people have been saying, if your content is shit, buying votes won't make it better.