That's a very good question. Whether it's profitable or not for the borrower depends on a lot of factors; for example if the price of Steem and SBD goes up or down during the lease period, and how the additional SP affects your curation rewards. It's hard to predict ahead of time. Even if you don't make a profit, there are many intangible benefits such as increased prestige, additional followers, and additional influence on the platform. Some people use borrowed SP to support their local ethnic communities or run contests that would be impossible with less SP, and look at the lease price as an investment in their long-term Steemit success.

If you are considering borrowing SP with Minnowbooster, I think it's important to first make a plan for how you will use that SP. If you want it simply for the sake of trying to make profit through curation rewards or excessive self upvotes (which the community generally frowns upon), then you might be disappointed.

Remember you can also set your own price, so you control how much you are willing to pay. If you place an order with a low APR, you might have a harder time getting an investor to fill it, but on the other hand you'll have a better chance of making a profit.