Id be looking at the small print for "fully let" and see when exactly 6% is available. Could be your standard guaranteed rent trick. Whats the interest rate on the finance? If the 6% net return is cast in stone for the long run and guaranteed by contract, and the company offering it is very liquid and insured, of course it would be fantastic because your paying your mortgage off and probably making some profit. But Id have serious doubts about the 6%