I've previously worked with PCB123 through my job, and found the experience to be good. Their included software was easy to use and in no time I had my first PCB designed and ordered. I was really excited when the finished PCB arrived in the mail, which turned almost immediately to disappointment when I found out I had screwed up the dimensions and it wouldn't work. i redesigned the PCB, printed out a true to size version of the layout, confirmed the sizing, and ordered again. I was happy to find out the second one was the correct dimensions, and after soldering, it worked well for its purpose.
For my personal project I drifted more towards Eagle from CADsoft as there are several batch PCB houses that will manufacture the PCB for fairly cheap, if you're willing to wait a bit longer. The PCB houses all accept files from Eagle to manufacture your design. The last time I checked, you could get the same Gerber files from PCB, but it would be $50. You can use Eagle for free as long as you're not going to use it for a commercial venture, so I gave it a shot. PCB123 delivered me a great board in a short time, but was also rather expensive compared to what I wanted to spend on a personal project where time wasn't really a factor. I just checked their rates again, and seem to be similar, but still more.
Once done with my design, I ordered through OSH Park. I'll update once the boards come in.
In the meantime, here's a link to the PCB design reference page.