Bananas are wondeful and nutrient rich, however they are also incredible packed with sugar and have a very high glycemic index (the sugar in them is readily and quickly available, and raises blood sugar significantly over a very short time) unlike most foods typically considered complex carbs (which tend to cause more prolonged but less intense increases in blood sugar).

Definitely not recommended for diabetics or others concerned with blood sugar spikes. In general, a banana every other day or when craving may be wise unless your carb intake is other wise well controlled.

I'd also suggest eating greener, under ripened bananas - the nutritional content is the same, and the less ripe a banana you find palatable the more fiber and fewer sugars and longer it takes to digest that sugar into the bloodstream (this the lower its glycemic index rating and effect on your total sugar intake).