Now that I have experience with three PCB manufacturers, I would like to share my impresson of each of them.  The tl;dr is that having tried OSH Park, Seeed Studio, and JLCPCB, I will manufacture my boards with JLCPCB in the future.

Full disclosure: JLCPCB offered me a free batch of PCBs in exchange for writing a review; however, they did not influence this review, and what follows is my honest opinion based on my experiences.

Ordering Experience

OSH Park has a very user friendly website and allows uploading KiCad and Eagle PCB files directly so you don't have to convert them to gerbers first.  They also make it easy to share your designs for other people to manufacture. This makes it much easier for a beginner to place an order.

Seeed Studio's website is considerably less friendly than OSH Park's.  I had intended to order my second batch of boards from them but I encountered a problem with their online gerber viewer which made me nervous. The drill holes for all of the pads were not showing up in the correct place.  I emailed Seeed Studio about this and they said it was a bug in their gerber viewer, and to go ahead and order my boards since someone would review the layout for any problems before manufacturing.  However, the experience didn't inspire much confidence, and by the time they got back to me the next day, I had already placed my order with JLCPCB.

JLCPCB's website is also not as easy as OSH Park's, and requires generation of gerber files, but the ordering process went smoothly and the images from their gerber viewer looked correct. JLCPCB is also affiliated with EasyEDA, a web-based PCB design suite.  I have played with EasyEDA some and it seems like a good tool, but all of my final designs have been done in KiCad.  Strangely, I could not find any way to directly submit PCB designs from EasyEDA to JLCPCB for manufacture, which seems like a natural advantage they could offer. But unless I missed something, you still have to generate gerber files and upload them.

Price

OSH Park is much more expensive than manufacturers based in China.  A batch of 3 boards cost $38.70 with free shipping. JLCPCB charged me $5 for a batch of 10 boards, and they had a special on my first order of 5 boards for $2. Even with $18.47 shipping from China factored in, it was more expensive to order 3 boards from OSH Park than was to order 15 boards with 2 separate designs from JLCPCB. Because of the expensive shipping, it makes sense to order multiple batches of boards from China at once if you can.

This isn't a completely fair price comparison, since the JLCPCB boards were made with a leaded HASL finish versus OSH Park's more expensive gold-plated ENIG finish. JLCPCB offers ENIG, which was still cheaper than OSH Park, but much closer in price.  However, OSH Park only offers ENIG so you have to pay the premium for it even if you don't want to.  

Seed Studio also offers HASL and ENIG options, and in general was slightly more expensive than JLCPCB for a comparable board. Since I didn't order from them, I don't have a record of the exact prices quoted.

Lead Time

Even though OSH Park is located in the US, my boards took longer to arrive than they did when I ordered them from JLPCB in China.  With OSH Park, I placed my order on Feb 15, the boards shipped via USPS on Feb 26, and arrived on Mar 1 (total of 15 days).  With JLCPCB, I placed my order on Jun 14, they shipped via DHL on Jun 19, and arrived on Jun 21 (total of 8 days).  Since I never actually placed an order from Seeed Studio, I can't comment on their turnaround time.

Board Quality

JLCPCB and Seed Studio offer a choice between leaded HASL (the cheapest option), lead-free HASL, and gold-plated ENIG (the most expensive option).  OSH Park only offers the more expensive ENIG option. I will refer you elsewhere for a discussion on the finer points of ENIG vs HASL.  I have not ordered ENIG finish from JLCPCB or Seed Studio, so I can't offer an apples to apples comparison. What I can say is that I found the quality of the HASL finish to be just fine. The pads are solidly attached and they take solder easily.  It is certainly more than good enough for making a few boards for a hobby project.

Although I have not ordered from Seeed Studio personally due to the problem I ran into with their gerber viewer, a friend had them manufacture some z80ctrl boards manufactured from my design and sent me several of these boards to evaluate.  Overall I found the quality of the PCBs manufactured by Seeed Studio and JLCPCB to be very similiar.

Aside from the finish of the pads, one major difference I noticed between OSH Park and the other two manufacturers is the way they finished the edge of the PCB. OSH Park's boards have perforated tabs that connect the boards into a panel while they are manufactured. These tabs have to be broken off with pliers and this leaves rough spots around the edge of the board. If you have traces near the edge, you have to be careful not to damage them when breaking off the tabs.  JLCPCB and Seeed Studio's boards have a much smoother finish around the edge of the boards with no tabs left on them.

I also had a manufacturing defect on my boards from OSH Park that did not occur on the boards manufactured from the exact same layout by Seeed Studio. The solder mask didn't completely cover the DTR trace near the edge of the board, causing a solder bridge that shorted between the trace and a neighboring GND pad. This prevented the AVR reset circuit from functioning correctly and could have damaged the DTR signal on my USB-to-Serial converter.  I think this was partly my fault, since in my first revision of the z80ctrl board, I ran the trace from the DTR signal very close to the edge of the board.  In later revisions of the board, I rerouted the trace so it is further away from the edge of the board, but it is a point in Seeed Studio's favor that even with the trace near the edge of the board, their boards did not have this problem.

Conclusion

I really like OSH Park's ordering experience, but Seeed Studio and JLCPCB definitely win on price and turnaround time.  Also, in some ways, I feel that they produce higher-quality boards than OSH Park.  Seeed Studio needs to work out the bugs on their gerber viewer so that people can trust it when ordering their boards.  JLCPCB's website is not as easy to use as OSH Park but it works well, and provides plenty of documentation to help you figure out how to generate gerber files from the various CAD packages.  All in all, I think all three manufacturers are a decent choice for hobbyist PCB makers, but after trying all three, I plan to continue manufacturing my boards with JLCPCB in the future.