Actually, no it isn't. While bananas are a good addition to your overall food plan (they contain potassium, and some fiber, Vitamin A, C, B6, and magnesium, but they also contain sugars) they wouldn't be good as your only food source (unless you're stuck on a deserted island that only has banana trees ... better bananas than starvation, right?).
This is where we talk about the "diet" ... the "Banana and Skim Milk" diet. Oh yeah, there was such a thing and in fact, it can still be found today all over the web.
On it's own, it isn't a healthy diet plan for long-term use. But does it work? Yep, sure it does if you stick to it. Why wouldn't it? It reduces your calorie intake, like ANY working diet plan, but it is so not fun, not even if you love bananas. At the end of it, you will not only
not love bananas any longer, you'll
hate them. Hate the sight of them. At least for a while.