I just bought the Image ONE's a couple of weeks ago, my first foray into higher quality headphones. At the same time, my brother bought the Shure 750DJs and so I have had a fair amount of listening time on both.
There are a few big things that make these headphones very different. The first is the size and portability. The Image ONEs are much smaller than the 750DJs, and fit the head in a much different manner. The cups on the ONEs are smaller and a different shape, the band is thinner and narrow and fits more snuggly around the head. That being said, I do not find one to be more comfortable than the other, they just fit differently.
Secondly is the cabling. This isn't a big deal, but some people gripe about it. The 750DJs have a one side cable (I don't remember which), and is thicker and longer (10ft I think). The ONEs have a shorter and thinner cable, built-in iPod/iPhone controls and is connected to each ear piece. Something worth noting.
Due to the fit of the ONEs and the size, I find them to be much more portable than the 750DJs and they also come with a nice hard case for easy travel.
Now for the (arguably) most important factor...the sound. I will start with the Image ONEs, since I know them best. The Image ONEs have a clear and defined sound with an emphasis on the low end. They are by no means overly bass heavy, but it is definitely prevalent. The mids are fairly clear and have good strength behind them. The highs seem to be very dependent on the recording. In many cases, they can feel somewhat veiled. They are still there, but often a little too flat for my liking. However, this could just be due to the closed nature of the cans.
The 750DJs have a bit of a different sound to me. They are much more bass heavy, but they still remain clear and defined. For my taste, I feel that the bass response is a little overpowering, and it leads me to an opinion of a lack in mids. I feel this way because it seems as if the bass will sometimes overpower the frequency range slightly above it. This is very dependent on the music though, and different genres will experience different things. I find that the highs in the 750DJs are a little more emphasized and snappier than the Image ONEs which can be a nice addition as well. The soundstage feels much larger for these headphones, but they also completely fit over the ear unlike the ONEs.
Overall, I really love my Image ONE cans. I prefer them over the 750DJs, but not by much. I enjoy their sound and their mid/mid-low response and feel like they are a lot of fun to listen to. They seem to be a little easier to drive by my iPod/laptop, and the 750DJs can benefit a little from a small amp. Despite this, they still work really well without a dedicated amp.
I listen to mostly grunge rock, metal, progressive metal, and things similar to these categories as well as rap/hip-hop on occasion and classical thrown in there as well. Were I to listen to more bass heavy music and a little more rap/hip-hop then I would probably prefer the 750DJs over the Image ONEs.
