Energy Transfer is offering a huge 7.5% yield that is well covered, but there are other issues to consider before you buy it.If you're looking for a high-yield investment to add to your portfolio, you will definitely find Energy Transfer's (ET -1.38%) lofty 7.5% distribution yield to your liking. But can it set you up for a lifetime of income? That's a harder question to answer, given the history behind this midstream master limited partnership (MLP). Here's what you need to consider before you buy it.What ...