I use them under a few of my suit jackets or in other layering situations, especially in the winter, when maximum clothing with minimal bulk is required.
Alone, they can look unfortunate and very early 1990s, and they are hard to wear if you have a large bust (uni-boob is generally the result). Because I have a long torso, I had to cut my bodysuits just below the hipline and add in another strip of fabric to extend it. The extension is hidden beneath my skirt or trousers, but it's a point to be conscious of if you have a long body or are at all tall.
So my estimation of a bodysuit is that it can be convenient, comfortable, and attractive in some applications, but care must be taken to not look like an unfortunate "Sweatin' To The Oldies" throwback, or to flatten your bust, give yourself a wedgie, or show off bulges you'd rather conceal.
For the record, the ones I use the most are a cream colored mock-turtleneck style, a black one in that style, a black leotard, and a wine-red leotard. I wear them under jackets or other tops, and I have had to modify all of the above to fit my long torso.
NEVER wear it alone! It looks ridiculous. It can, however, look really great is layered. I have some dresses that are a bit see through, so I wear my American Apparel unitard underneath. And it's also nice during cold Canadian winters. Very convenient!