From my recent experience working with it today I can confirm that it works exactly as advertised in regard to it's ability to upvote your posts and it actually gives you a higher return than the amount spent.
How does MinnowBooster work for Upvotes?
According to documentation found in an article released by @minnowbooster
"To request upvotes from the @minnowbooster bot just send SBD (at least 0.1 SBD) to
@minnowbooster and receive an upvote worth up to 5 times what you paid. Add the post or comment url as memo.
At 100% voting power the bot will give your nominated URL an upvote worth 5 times what you paid in, with the received vote value decreasing proportionately as VP decreases, ie 80% VP yields 4 times and 60% VP 3 times, right down to breakeven at 20% VP and a loss if you pay in when the bot has less than 20% VP. (These are gross percentage values, please take into account that on Steem 25% of the post value goes to the curators, in which case the NETT breakeven is actually at 26.67% VP)"