Received the Moondrop Chu this afternoon and couldn't help but listening to them.
The short version:
these are well worth the $20. Heck, I think the sound is competitive against anything under $50.
Now, a bit more details. The accessories are good. Spring tips alone make these a good value. More on the effect of Spring tips on the sound in a moment.
General build quality is good. The shell is built as well as the Aria shell. Heck, the design and venting mechanism of the inner-shell is very similar to Aria. The fixed cable has a thin strain relief but it is the most vulnerable part of the IEMs. The cable itself is good, fairly supple, doesn't kink easily, and easy to untangle. I do miss the chin-slider. The carrying case is flimsy though, offering zero protection.
Now, the sound is dependent upon the tips used. With the Spring tips, you get a smoothed-out presence region. This results in a calmer, somewhat diffused presentation. Not the most engaging but very inoffensive.
If you want to inject a bit more life into things, try changing tips. The Spinfit CP-100+ for example brought up lower-treble just enough to make things more engaging. This peak can become fatiguing in some tracks but your mileage may vary.
The general sound-signature is fairly neutral. The sub-bass boost isn't that noticeable when you're listening to them. The graphs actually show more bass emphasis than what I hear. I would have preferred a bit more mid-bass body as well. Snare hits sound anemic and lack the authority I expect them to have.
Soundstage is above-average, whereas imaging is strictly left and right. Then again, these are concessions you make for much higher-priced IEMs so I'm not gonna be too critical here.
I also compared them against Aria and I am quite confused as to why some label these as "Aria killers". Aria are superior in every single aspect barring lower-midrange or male vocal emphasis. I find the Aria too recessed there in comparison. But in all other aspects Aria showcase why they cost more.
But for the price you pay and the performance you get, I think Moondrop Chu are a great purchase for those seeking a neutral-ish tuning in the ultra-budget range.