It is always nice when someone appreciate your work, especially if you give the product to public as free to use. It happened to me as well, I was asked to review a PCB fabrication house called pcbway.com. Without too much hesitation I have sent them Gerber files of OHS gateway, and two of weeks later postman brought me the package, nice :).
I must say I have tried around 4 different China fabrication houses, and this one is fifth. I tend to do my board easy to hand solder, and I do not use fine traces or gaps bellow 10mils. This give me better trust to choose rather inexpensive manufacturing processes. There is not too much difference between ordering in pcbway and other houses, you do all as usual on their web pages. One difference though, is they probably inspect the gerbers by person. It came to me as I have used 6mil trace/gap option as the basic available, and within 1 day after submitting gerbers they proposed me to change it to 5mil. First time I've seen that actually someone had a glimpse of my layout before manufacturing. Of course it was a little more expensive, but still within the free coupon I was given for review.
The boards themselves are nicely made. I can compare to 2 other PCB houses I have boards from. I will skip the traces evaluation since I have no visual or other tool to do any reasonable comparison. But first think you notice with bare eye is silk screen, it is noticeably vivid and precise. It's placement is usually the weakest point in China PCB houses. But this board is done accurately, not faint and not smeared. I think using this fab one can afford to put logo on silk screen and not be afraid that it will come out ugly.
Second difference is via placement. When you use IC with many legs you usually route signals to other IC close to each other, this leads to having vias sit to each other in some pattern too. On such patterned vias you can easily spot the sharpness and bit placement. I must say there the vias on the PCB are great I would not be afraid to go with lower then my 0.3 millimeter. Along with via quality goes actual hole placement. And it is similar to vias, nicely placed in exact center of pad.
Third difference I've noticed is solder mask, using loupe the mask is somehow more corresponding to what is draw in gerbers. Put it simply, exposed copper pads are smaller and more precise. This will help especially during soldering, the small components will not wander around, and IC will not have as many solder bridges.
Here is picture hopefully supporting my point, with pcbway PCB closer on top overlapping seeedstudio PCB. You can see here of-centered holes and wider solder mask along with nice silk text.
As I said on beginning, I do not develop over-complicated PCBs with many layers, buried vias or impedance control, so my evaluation is as erudite as hobbyist can offer. But I would say I will use the pcbway services in future for sure.