Their only advantage: They save you from carrying around two pairs on the very few times you actualy need to see in both dark and bright areas.
Disadvantages:
1) Less choice in color or darkness. With regular sunglasses, you get tons of choices with color and any amount of darkness you want from 10% to 90%. With transitions, you get what you get and that varies with many conditions.
2) I wouldn't go polarized with transitions lenses. Polarized glasses suck with LCD monitors (and other polarized things like cell phones) since you might have to turn your head 90° to use a computer, cell phone, camera, etc. So, pretty much, you can't go with polarization on your main glasses. But polarization is exactly what you want in a pair of sunglasses - to stop the glare of the sun (directly or indirectly as it bounces off water/snow/mirrors).
3) You pretty much want small stylish lenses on your regular pair and large wraparound lenses for your sunglasses. Transitions either suck at one or both of those criteria.