I have run in the phones for a while and lived with them for a bit. Not a review but a few impressions...
I think that Senn have done themselves proud for a CX300 replacement with the CX3.00. It should do well for them.
The CX3.00 has nearly the perfect physical design. Small, very light, perfect angle of the sound tube, and short relief. You almost forget they are in. Very comfortable. Can be worn both up or down. You can even dial in a bit more or less seal with a little bit of twisting when worn down. CX5.00G is a slightly longer housing but with larger arms coming off of it is a fairly shallow fit. Not really necessary but probably just to differentiate from the MIE design. The arms are easy to grasp for insertion and removal but don't give the ease of fit or comfort of the CX3.00 design.
The cable on the new CX phones is a mixed bag. Nice look and feel and the oval design does both feel good on the fingers and prevent tangles nicely. The downside is the material used for the sheathing. Kinks from being wrapped around the case insert remain on both my CX3.00 and 5.00G. This sheathing also doesn't like cold weather and stiffens outside in lower temps. Worse for the CX5.00 with their longer stems which help to start unseat them if you turn your head as the cable really doesn't want to flex any more.
Not perfect though. The CX3.00 is a bit consumer sounding which is probably tuned the way they want. A bit thick and bassy. Teamed with a lean source, the right tips, etc. they are quite capable though. I do recommend a loose fit as well to both give a quick fit and lessens some thickness. Striving to get absolute best seal is best for those who want the biggest bass and meatiest sound.
In stock form the CX5.00G is a better sounding version of the CX3.00. A bit deeper sounding with the housing and a bit tighter and cleaner. Both are warm and bassy, slightly bass tilted sound that sloped down towards the treble. Rich and weighty sound that is forgiving and pleasing.
With the most Head-fi tweaking to get them to sound the best for me(CX3.00 allows this best due to more fit adjust-ability ), this driver is quite capable. More so that the previous one in the CX985 and others. I hear better mid-range and less sharpness and peaks in the treble. Can be fairly large sounding with good 3D sound. Pretty quick and aggressive with good energy and dynamics. High fun factor and engaging sound is there. Haven't heard this quality and likable vocals yet from a CX. I do like them best when the warm blanket is removed to reveal the most ability otherwise they are good and likable but more consumer sounding then I like. The recipe I like is lighter, brighter for everything else in the chain plus the CX's equals the best result.