While I wouldn't say that the X5 iii was bland, it didn't have a very engaging tonality. It has a smooth, easy-going sound that's generally good for most genres. However, I didn't feel that it was very prominent in any single one department. For the price and feature set, though, it's a pretty decent buy.
We've all got different takes on gear. What I will say is that the original X7 and then X5iii are what convinced me to take FiiO off my radar. X7 was buggy, had poor GUI add button placement, and I didn't like the amp module thing despite initially thinking it was cool. X5iii looked cool but didn't find the sound engaging at all, still didn't like the GUI much, and didn't like the volume wheel implementation.
On the other hand, I've found sound from all the Shanling DAPs engaging despite distinct sound signatures, GUI on all but original M3 is the best Hiby implementation I've seen yet, and ergonomics are great.
I also really enjoy the Cayin DAPs I've used. They also have engaging sound, but the GUI falters a bit. Not as organized as Shanling and a bit cluttered.
As with you I've used many other DAPs, but I see FiiO and Cayin as the main competitors so I'll stop there with the comparisons.
In the end it comes down to personal preference, and Shanling's formula of engaging sound + streamlined GUI and ergonomics keeps pulling me back to their DAPs.
M5S seems to be a Android-based Dap.
I knew about it for a while. I was just playing it cool because I didn't wanna leak any info about it. It's fine.
Shhh 
But seriously, following up a bit on what I said up above...
After the original X7 was released, I followed up quickly with Cayin and Shanling to urge them to develop Android based DAPs. Of course, I want telling them anything they hadn't already thought of, lol. And obviously, Cayin and Shanling took different approaches. If you've followed the transcription of FiiO, then Cayin, and finally Shanling, you'll see that they do indeed take different approaches.
FiiO established early market dominance with X3 and X5 so they've got the best name recognition. They're the first ones the majority of headfi'ers look too when checking out new DAPs. Since they been around longer, they're experimental with new tech like Android and modular amps (yes, I know they're not really new tech, just new tech for mainstream DAPs). So even though I (and a few others with some credit) are vocal about our opinion they're not keeping up with Cayin and Shanling as far as sound quantity goes nor are they keeping up with Shanling as far as overall user experience goes, FiiO is still dominant.
Cayin came second and have IMHO focused primarily on nailing down the best sound quality they can get out of their components. I've been very happy with that aspect of their DAPs. Having gotten to experience a couple from beta through final product, I can tell you that they do a really good job with optimizing sound through FW updates. They make bold design choices which can really work (N5, i5) or, well, not so much (N6). Where they fall down for me is the GUI. It's just not streamlined enough. I've offered my suggestions for improvement, but they obviously have their own take on GUI that doesn't quite mesh with mine. IMHO, if they'd come out with N6 + N5 earlier and with improved GUI think Cayin would have stronger market position right now.
Like Cayin, Shanling has been in existence for a long time. They're just the new kids on the block when it comes to DAPs. The original M3 had bold design, kinda like a giant Zippo. The thing was a brick and had an antique feeling GUI, but it did sound really good and was the DAP of choice for Suyama-san (fitears iem designer), which says something for people in the know. I mean, Suyama-san commissioned a picture of his favorite anime character listening with M3 plus FitEar IEM. M2 and M5 modernized the look (both physical and GUI) and scaled down the size but still retained a sense of uniqueness. M2s and M3s again changed the design language in a similar way as the original M2 and M5 by scanning down the size further and modernizing the physical design. GUI remains basically the same, which is a good thinking because it's great. With all the praise in healing on Shanling, you're writing why they're not more popular...
Two things have held Shanling back. The first is a rookie mistake. They released specs early on which indicated that their DAPs had high output impedance. That didn't make sense to me because I was measuring IEM and saw no difference in FR Curves for multi-BA IEM. Shanling did figure out that they weren't using the same measurement techniques as other DAPs makers and issues correction which indicated that most of their DAPs have very low output impedance. That initial faux pas scared off many reviewers, so Shanling why given much notice on HeadFi. The second thing that's held them back is market dominance primarily by FiiO and then secondarily by Cayin. It's tough to be the new kid on the block. However, I think M1 started to change that. It was small, cute, had good GUI, and was a great transport for higher end DAC/Amps like Mojo. People took a chance on it and liked it, and I think that had a halo / spillover effect on the M2s and will continue that with the M3s. People are starting to take notice, and IMHO the future is bright for Shanling.
Just my two (very rambling) cents...
