Your opinions about KOETSU URUSHI BLUE or VERMILLION

Well the differences in Koetsu builds are always shrouded in a bit of mystery. I don’t have any experience with any Urushis or the Black, but I do have experience with the RSP and a number of stone bodies. The differences in stones like Jade vs. Onyx are pretty subtle and hard to even describe. The magnet type (Platinum vs. Alnico - I have the Alnico from an older Onyx Gold/Signature I had rebuilt) has a much bigger sonic impact. The Alnico magnet also has double the output level. The cantilever, diamond vs boron also has a much bigger sonic impact - the DC increases detail, clarity and dynamics without any obvious downside. The RSP (it has a clear lacquer, which I believe is missing from the lower Rosewood models) has less bass impact and power vs. the stone-body platinums, but the magnitude of this body difference’s impact is certainly less than that of magnet type or cantilever. And then you have the top-range Coralstone and Blue Lace stone models, which even with boron cantilevers seem to be a little "better" than the lower-ranked stones somehow, but it’s hard to say if this is due to being granted the "best" hand selected motors, and/or possibly getting slightly larger magnets (they seem a little bit higher in output, too) - no idea for sure, though.

In picking Urushis, the big choice is Vermillion vs. anything else. Again, I’d advise a proper SUT for the Vermillion (I like SUT’s with all Koetsus anyways, but that 0.2mV output will be especially tough without a SUT - and the Hashimoto HM-7 are fantastic with Koetsu, highly recommended). If you choose the non-Vermillion Urushis, just pick a look that pleases you! I think all the Urushis are beauties, and I'd love to try one someday. Probably would not be a Vermillion though, because even with a good SUT noise floor can creep in at that level.

In fact everybody is talking, but no one knows really how a vermillion sounds.....

Just curious, why Koetsu is so expensive, what is the technical advantages? I remember Garrott brothers, who rebuild them and retipped them for the clients, often commented about poor build quality of those cartridges (made at that time by Sugano-San himself). Now it's not even Sugano-San ...  

Even lower model are extremely expensive compared to many great cartridges with reputation. 

I have never owned any of them, but i wish to know 

Mulveling, The Rosewood Signature Platinum weights 3 to 4 grams less than a stone bodied cartridge and requires a heavier tonearm to get the same bass impact. Bass and "impact" are the two most sensitive characteristics of a cartridge, the easiest to mess up. Resonance tuning is a real issue here. I think a lot of people miss out on it because most systems just do not go down that low. Not trying to insult anyone but it is hard and expensive to get a speaker/room to go below 40 Hz effectively. I had to build my own house to get it right. Anyway, the RSP bops just fine if you get that resonance frequency down under 10 Hz. I have never compared it directly to a stone bodied unit but the motors are the same and the only real difference is the mass of the cartridge which can be compensated for. Trying to put a stone bodied Koetsu in an arm as heavy as the 4 point 14 would be asking for it. The arm would go airborne with the first warp. I believe the stone bodies came along to make the cartridges perform better in the lighter tonearms that most of us use which are compatible with the majority of cartridges that are of medium compliance.
Chakster, they charge that much because they can get away with it. There are also several middle men. I think Ortofon has much better quality control. My Ortofon Windfeld Ti will track circles around my Koetsu and is by all accounts more neutral. 

@mijostyn 
I've run the RSP on a Fidelity Research 64fx, an FR 64S with a 20g Ikeda headshell (that's more than enough mass for anything), and a Graham Phantom II Supreme. I think the stones are better.