I just got a new pair of SE535's back from Shure as a replacement for my broken E5s that were on back-order and thought I would post my comments here since I had read a lot of good information here prior to deciding to accept the upgrade replacement. My background is that of a part time professional audio/video engineer and musician with 14 years of experience, so I originally purchased the E5s for their balanced performance as a portable studio quality monitor. Also, keep in mind I literally just got back from the UPS depot with these and was listening to them on the way home with my wife driving, so this is very much first impressions.
The sound on the SE535s is definitely far less "textbook level" in comparison to the E5s, though I'm not sure this is a bad thing. I noticed far better response in around the 200Hz to 600Hz range with a much richer, cleaner texture. (Something which I thought the E5s were lacking for drum and bass work and even some male vocals.) On the flip side however, the high-mid roll off is definitely apparent. Probably somewhere in the 4 to 6kHz range, there seems to be a sharp drop (though detail is still clear if muted). It seemed to improve somewhat as it moved further up but the higher range was still probably a hair more subdued than the E5s. My assessment of the overall sound is that while the sound may be less textbook, with some practice to get used to the sound and correcting for the response curve, I still hear plenty of detail in the high mids to be able to work with it without straining and the cleaner low end is definitely welcome. If you are primarily working with female vocals, the 425 (supposedly similar to the E5s though I have not heard one myself) is probably a better choice (and cheaper), but I do like the sound of the 535s for more bassy instruments and vocals provided you are comfortable adjusting for the response curve.
The 535s are far smaller than the E5s. I was shocked by just how much smaller (I'd guess at least a third smaller) though if you use the universal fit kits from the E5s, the silicon fitting I was using appears identical in size to the new ones (though the canal is significantly smaller, so no using the exact same ends.) The connectors to the drivers themselves feel quite sturdy (like a heavy snap) and swivel easily for positioning but not too freely. Not sure if that will change with wear. The ear wire does not feel quite as sturdy as the E5s, however being so much lighter of a set, I'm not sure that this will be an issue and they seem to be fine so far. The 1/8" jack seems similarly constructed to the E5s but has a more streamlined case. They also added a nice lip to make unplugging a little easier without putting pressure on the cable. (Which is what did in my E5s.) The crossover (assuming it is even still on the cable (it is now not transparent even on the clear set) is much smaller and the drivers themselves have far less space for resonating though this does not seem to be a problem. Also, it may just be a factor of the smaller chassis, but the drivers themselves seem larger.
While not a strictly flat, analytical set. The SE535s maintain a solid, usable sound for recording, mixing (provided I can still pop them in and out easily) and general listening. As someone looking for a pair of studio quality headphones I can take with me and use whenever I need a good set of monitors they don't disappoint. They fix many of the issues that were problematic with the E5s. I'm not sure that I would purchase them over the $300 SE425 for general stage monitoring if I hadn't't gotten them as part of a replacement offer, but for bassy vocals and instruments it could very well be worth it. Also, for a nice pair of hybrid in-ears that can be used for both professional work and casual listening, they hit the spot nicely.