Borrowing to invest can be effective if the investor has a solid financial backbone and the wherewithal to hold investments and not sell in a panic if there is a downturn.
I agree with previous posts that priorities, before borrowing to invest, should be 1) clearing all consumer debt; and 2) maxing out registered tax-advantaged accounts, including RRSPs, TFSAs, and RESPs. Getting a grip on mortgage debt depends on one's perspective; some prefer to invest as a priority before paying off their mortgage, others would rather clear the mortgage and then invest. There is no clear cut answer here.
I personally did borrow $35K in March from my HELOC at prime to purchase the XDV ETF (among other things), with a nice dividend yield and some capital appreciation over the past few months. This only represents about 7% of my net worth, so there is very little risk, and the dividend yield more than covers the interest. Since I have no other debt of any kind (and no mortgage), I will gradually pay off the HELOC a few hundreds of dollars at a time, making the income "clear" at some point.