Hi, just saw your comment. Sorry for late replying... I don't have SBD at the moment, this why I haven't use them yet to this post. For my side, they are profitable... All you need is transfer SBD to their account and paste your ulr to memo field while you are sending SBD.

Please check following url's to learn their limits

https://steemit.com/minnowbooster/@minnowbooster/minnowbooster-update-log-15 for @minnowbooster

and https://steemit.com/steemit/@randowhale/how-to-use-randowhale for @randowhale