Have the SON hub dynamo on both my bikes, and (having ridden it through streams and across beaches) swear by it. With a 60lux Cyo up front, it beats back all but the deepest darkness, and regularly keeps up with the Hopes and beams of the FNRttC.
Actually on beams, it throws a very useful shape onto the floor ahead (ever present in the base of my flickbook vids), letting you catch potholes / animals / bedfellows well in advance.
Is it brighter than a very good MTB light with a fully charged bottle mount battery? No. But two hours later, it'll more than hold its own. 40 odd hours of unexpected night riding later, on unlit (and sometimes unmade) roads, without access to a power socket... Its starting to look very good indeed.
Flip side? Technically, I suppose it must add some drag, and it is notchy at very low speeds. There's a weight penalty too (minimised by its position in the centre of the wheel) but I should think it compares very favourably to a stack of AA batteries required to keep a similarly bright cateye alight.
IMHO, SONs are not comparable to bottle dynamos, and compare extremely favourably to other hub systems out there. I wouldn't be without mine. Its not that its always charged. Its that its always at full power.