Well, I auditioned Senn IE6 briefly at a local audio store a few days ago and compared them with my IE8. My source was unamped Sony NWZ-A816. My first impression was the size and weight of the IE6 - compared to IE8, it's much smaller and lighter. Then, once I started putting it into my ear, another thing that I noticed immediately was the fit - it was much better than that of IE8 for my ears! I used the medium, soft black single-flange silicone tips and with the IE6s the fit was very nice and secure, providing good isolation. By contrast, I am having a much harder time shoving the large, heavy IE8 drivers into my ears using the same tips - with the IE8, I have to use foamies to get the same secure fit as with IE6 and even then, they are not as comfy as IE6 and the sound quality suffers.
As for the sound, compared to IE8, the IE6s sound quite a bit thinner, less detailed and less accurate. However, compared to Senn CX300 and other cheap canalphones like that, IE6 are, of course, a big step up. They offer a nice tight bass response that's way more accurate than that of CX300 and the quantity of bass is not overpowering like on CX300. The mids and highs are also very good - they are much clearer and cleaner than CX300 mids and highs and offer more detail as well.
Overall, I think that IE6 is a very good IEM, if you can get it for around $150. It provides a more spacious and more forgiving sound than balanced armature IEMs, while sacrificing some accuracy and detail. However, at it's MSRP, it's not quite as great, since for the same price you can get a more accurate and detailed Sleek SA6 or Head-Direct RE0. Nevertheless, if you are looking for easy insertion, best comfort and dynamic sound, the IE6 is still worth considering even if it cost $200 or slightly more.