Fixed rate interest only mortgages.

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What's peoples opinions on fixed rate interest only mortgages? These are loans that are interest only for a period of time (10 to 15 years), then switch to principal + interest. These loans are usually ARMs, but I've recently become aware of lenders offering these at fixed rates too.

Theoretically, these loans would let me buy more house now than I could with a standard 30 year fixed loan. I pay tax deductible interest for 10 years, and after 10 years I can either sell or do the principal+interest payments. In 10 years, in theory I should be making more money from the Day Job so I should be able to swing the principal+interest payments. Also, during those 10 years I could work on getting the principal down if I got a couple of roommates to pay the interest charges. This type of loan sounds great to me. I like being able to pay just interest or principal+interest. So whats the catch?

On a related note, how is this better (or worse) than a 40 year fixed or a 50 year fixed? Do they even have 50 year fixed? I've seen 40 year fixed, but not 50 year.