The dual element ones from Nikon (3t-6t) or Canon (250D, 500D) and also from Minolta are high quality. The single element ones were not worth the effort in my experience.

Compared to tubes, they are lighter, easier to carry and don't mess with AF contacts which are required for some low end Nikon cameras. But on the flip side they eat into working distance more than tubes do. I'd get diopters for occasional macro use but get tubes and macro lenses if carrying them is not a bog deal.

-Anupam

PS: The new Macro forum would have been a great place to ask this.