Several days ago @kingscrown wrote a post called A New Bot In Town and I suggest you read that too, not only because @kingscrown is one of the more trustworthy people on Steemit, but because he knows infinitely more about these things than I do. I will say that he wouldn't push something that is a scam. With @randowhale seeming to have developed narcolepsy Steemit is in dire need of another reputable upvote bot (and I'm in no way implying here that there aren't others on here). Moreover, I keep getting notifications from the @randowhale notification widget telling me he's awake when he's not... of course the money is always refunded, but it becomes increasingly irritating.
I'm not exactly sure about how @spinbot works other than it "spins" on Wed and Sun. I'm not certain if how much you put in is cumulative or not... I suggest you read @kingscrown's post and @spinbot's posts as well. I can say that I received a "spin" from them and it seems to have boosted my post by around $3.00. I also put in for a spin for @kingscrown. I don't know if it helped him or not. Anyway, I like the concept and will continue to "feed" the bot. I encourage you to check it out for yourself.
