Full disclosure: I was contacted by a JLCPCB rep offering to do a few boards for me in exchange for a review.  I should further note though that I have done business with them as a normal paying customer in the past.

I used JLCPCB for my current board version for my Water Alert project. I also did a slightly updated board version of my Attiny Micro board, had them do a few additional boards for my Atmega328P Target, and for kicks some Sega Gensis/Megadrive cartridge PCBs from a design I found here.  

The ordering process was easy.  I really like that they show you images of the uploaded gerber files and in this instance it pointed out an issue.  For my Atmega328P Target board, I noticed that a portion of the silkscreen was distorted.  Under KiCad everything looked fine. I looked up their instructions for exporting gerbers in KiCad and saw that the include extended attributes option was checked in their screenshots.  I went back in KiCad and did that and resubmitted them.  The silkscreen issue went away.   

The PCBs arrived quickly via DHL for a total turnaround time of about a week (I normally use the least expensive shipping option that usually takes a few weeks, but since it was on them chose the fast option.) They arrived in vacuum sealed packaging (usually they arrive neatly stacked in the package, but it's not really a big deal.)

Overall the PCBs came out quite good.  Alignment was good.  The edges of the boards were nicely routed. Silkscreen text is nice and sharp (with the exception of R4 and R5 labels on my Water Alert board, although to be fair the text is a bit small.)  Order numbers were printed on the boards.  One thing that I'm really impressed is that JLCPCB is able to get soldermask between the pads of fine-pitch surface mount component footprints as shown on the Sega Genesis/Megadrive cartridge PCB (note that although I didn't specify gold fingers on the edge connector, it should be specified if you're doing something more than testing/prototyping/just tinkering.)

My recommendations to JLCPCB:

Give us the option to change the PCB quantity before the order is submitted.  JLCPCB changed their default quantity from 10 to 5 and I didn't notice until I uploaded everything.  Since one the boards had to be manually approved it would have been a bit of a wait if I just deleted the order and started over again.

Retain promotion offers when an order is cancelled. On a previous order, a free shipping promotion was offered.  There was an issue with a board I uploaded so the order was cancelled. When I re-uploaded with the issue corrected the free shipping promotion didn't re-appear.

In summary, I'll give them a thumbs up. I'd also like to thank them for offering to spin a few PCB designs at no cost in exchange for a review.  I currently don't have a regular stream of income at this time so saving anywhere I can helps greatly!

https://jlcpcb.com/