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Actually I thought usage of bot is like bearing loss,because sometime the ROI is unexpected and resulting in losses.

I've actually never lost money using an upvote bot. Of course, I look at the long-term return from using them. Keep in mind that you get both SBD and Steem Power from the upvotes you receive.. not just SBD which you used to pay for the upvote. Plus.. from using the upvote bots you may hit the Hot or Trending sections on STeemit which gets you more viewers which means more natural upvotes and more followers who will upvote you in the future.