I have used eurocircuits at many internships, and a lot of companies in The Netherlands have it as their favorite PCB supplier. Their lead time is short (8 workdays delivered for standard pricing), quality is good, and can do pretty decent rules on 2 and 4 layer boards.
If you go higher I am not really impressed. I got stories from a schoolmate where they required a 8-layer PCB for a USB3.0 oscilloscope etc. They had to resort to pcbcart because they could do better rules. Then again, they paid top dollar for the PCB's there too,
Eurocircuits and pcbcart are not expensive, itead and seeed are just stupidly cheap. You won't find it anywhere else.
But the lead times are horrible, and if you need boards faster you basically can't. I also hear sometimes complaints about etch errors on those boards, good you get 10 pieces.
Furthermore, if I do a 'professional' project and have to make a schedule for the project I want to make sure I get PCB's delivered in 8 days. So at eurocircuits I am confident I will get them delivered. In some cases software development must get started, and then 8 days can already be a long time waiting.
With itead it can take anywhere from 2 weeks to a month, sometimes even more. So that's another bit of extra pricing you're paying
Generaly for my hobby it doesn't matter.