In terms of technical skill, I would say that Ross is good, but not yet amazing. He is, however, getting better and better all the time in that regard, and I think he's reached a very good standard. His voice is stronger, more consistent and yet more varied all the time.
In terms of his vocal style, that's very much a matter of personal preference. Personally, I love the quality of his voice, and I think it's a huge part of Haken's unique sound.