That may depend - a lot - on which one you're considering, and also what you're attempting to shoot. I have a ring flash - one with flash tubes - and find that it is different than regular flash units, not really a replacement for them. Mine (Sigma EM-140) is also sufficiently big and bulky that I would definitely NOT call it more compact than an SB-600 in the shoe; in fact it's definitely NOT more compact than that.
The ones I think you're looking at are very, very low power - which may not be an issue depending on what you're shooting and from what distance. I would think that they would serve better folks who are using the short macros than the very long ones (eg a 60/f2.8 rather than the 200/f4), due to the fairly significant difference in working distance.
On the other hand, I have seen at least one "Macro LED ring light" that has quite a few LEDs and which is fairly powerful - but it is also even bigger than my EM-140, and I'm sure it does not help with the "circle of fear" for insects and other critters. It would even be getting in my way shooting model trains. I tried this particular one - I don't recall the brand - at a store and really didn't like the way it handled, which is one reason I ended up with the Sigma.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!