Just got some poster prints done using the online photo printing service PhotoBox and was amazed by the service and quality.
Poster Print: As you can see below, the poster print I had done was amazing quality, even at the large 30x20 inches size I had done here. I chose the 'matt' over 'glossy' finish for this particular photo and it came out very nice.
Original Photo Poster
It also lets you know what the quality is probably going to be like so that your prints don't come out too grainy or pixelated. The poster I had done above said it would be average quality but it actually came out absolutely fine.
I give PhotoBox 9/10 - Great value for money, quick to deliver and brilliant quality!