I used posterxxl when I think they were pretty new, they were offering some good deals so got some A3 size prints done. I thought they were good prints at the time (good few years ago) and looking at them again now they do look like VERY good prints

This was before I understood about sRGB and all that jazz, so just sent in jpegs from an un-calibrated monitor and the shots came back perfect, which was either luck or they do check what they are printing and get it right. The turn around was good (though this was a few years ago, they could be much bussier now I guess). They came from Germany, were printed the day after I ordered and shipped same day, got here a few days later.
For the 1-10 scale thing I think I would have to give them a 9 - only down side was it might have just been luck that I got good prints