IO loans are a very bad idea for one's primary residence or for people who can only afford a mortgage by using them, in my estimation. That said, there is another very good reason for the IO loan and it pertains to an investment (rental) property. If there is an annual loss on your investment property, you cannot take the tax advantage (depreciation) until after the sale of the property. However, if your accounting can show a break-even or slightly positive cash flow position, then you can take the tax advantage now. In our case, this makes a lot of sense because we are still working and need all the tax shelter we can get. Even so, I would probably not feel that comfy with an IO mortgage had we purchased our beach property in the past 2 to 3 years. For those who purchased earlier than that, your value to mortage, even in a slow market, gives you the comfort zone needed for an IO loan to work for you.
The worst misuse of these loans that I see are the young professional who wants to buy in an upscale market, but who cannot qualify under anything but the IO loan. They are typically purchasing a starter home or condo, in hopes of that property appreciating and escalating them into their dream home. In the case of expensive condos, particularly one must consider that it may take a very long time for that appreciation to occur. There are exceptions to every rule, of course. Good luck with your decision and purchase.