I thought I'd just chime in here and add my opinion since I own both the GR06 and GR02, as well as a couple other comparables.
First off, I really don't understand anyone that says the GR02 is better than the GR06. The GR06, to me, is clearly a higher quality IEM. It doesn't lack anything, it does everything well. Good bass, good extension, good soundstage, smooth midrange, nice highs, not too forward, not too laidback, good presence. And it has a better fit and a better cable than the GR02 (although I think that cable is fine). I don't think it excels at anything in particular, but that's a good thing to me. I don't like analytical but I want something lively. The GR06 just does that better than anything I've heard under $200 (haven't heard the GR07 though).
Now, the GR02 is a nice IEM. I actually like it a lot. It replaced my previous favorite fun headphone, my Fischer Audio Eternas. These Vsonics are just a class above. I LOVE it for the price of $36 (almost half what the Eternas cost, and I thought those were an amazing value).
I actually recommend the GR02 way more often than the GR06, because non-heaphone enthusiasts find it hard enough to justify spending $36. Sometimes $70 just seems insane to them. I WISH I was that way with sound equipment. Is the GR06 twice as good as the GR02? No. So for value alone, GR02 is the "better" IEM.
But, in my opinion the only thing that the GR02 does better than the GR06 is bass extension, and it's only slightly better. Yes, it has slightly more bass as well, but I don't consider that a plus. I prefer the bass quantity of the GR06 better. Some may like the extra bass the GR02 offers, although it's still no bass monster.
The GR06 is just a smoother sound overall. The more I listen, the more I appreciate this. If I could compare it to anything, I'd say it's like how the Sennheiser house sound is. It doesn't blow you away, but it makes you smile. GR06 also beats the GR02 in soundstage and highs. If the GR02 has a particular fault I'd say it's a grainy mid-high sound. It's quite noticeable when A/Bing the two phones.
So, for what it's worth, I'd say you can't go wrong with the GR02 for $36 or less. It's a crazy good value and a really nice IEM. But the GR06 is better, I think. If you have the extra cash I'd say spring for the GR06.