I had a Fiio X5 which I loved, and then replaced with an X5ii, which I love even more. I also have the Fiio Montblanc E12 headphone amp. My primary use for the Fiio products is for air travel -- paired with a series of Bose Quietcomfort noise cancelers, they've been terrific. I've got better phones I use at home, but the Bose are more than adequate when in the air.
I just ordered the M7 after careful consideration, and this is why:
Storage: I carry my entire music collection on the X5ii, using two 256 gig cards. That's an absolute requirement for me because I like to be free to choose whatever strikes me at the moment. The M7 will handle 512 gig cards, and the price for those has dropped dramatically.
Bluetooth: I've recently picked up a pair of Bose QC35 noise cancelers. They're not aptX, but they're not bad when paired with a good enough NON-LOSSY source, e.g. flac, which is how I store all my music. I can use them with my other Fiio products with a Taotronics Bluetooth transmitter, but it's an extra piece of hardware, albeit a small one, to carry with me. The M7's built-in Bluetooth, and support for a variety of Bluetooth formats, makes it preferable to X5. And, I can still use it with my wired phones and the E12.
Battery life: The M7 is up to 20 hours, 4 hours better than the X5. My longest flights are 16 hours, plus another few hours in the terminal and for gate holds. The M7 should handle the whole trip.
Size: the weight and size savings in comparison to the X5, though not dramatic, is still significant.
FM: Before I switched to Fiio products, I used to have a Sony iPod. The one feature I used which isn't replicated by my Fiios was an FM tuner. I liked to use it in airport terminals to listen to NPR. FM capability in the M7 is the icing on the cake.
The rest of the specs are comparable: I expect high-quality HD audio, enough power to drive my wired phones sans amplifier if absolutely necessary, and Fiio's usual excellent build quality. At $110 it seems an easy choice.