It's totally legit and good for getting over the garantía hurdle, but like Davidglen77 said, a lot of landlords won't accept this kind of garantia. Still, it IS a garantia, so it doesn't hurt to ask, especially since you are a foreigner. In my experience, landlords like to rent to foreigners, not from paises limitrofes, that is, because we don't know how lenient conviction laws are and how we can stay over in their apartment for a few months with nearly 0 consequence...and even if we knew that they know we are risk averse and have no desire getting into trouble with the law so far from home, for the most part. You never know, until you ask!