I’ve used them, and I’d recommend them for larger sizes, A4/12×8 etc. They apply some aggressive sharpening which is ok for large prints, but is too much on 6x4s. I did a side by side comparison between themselves and Photobox last year on smaller sizes and Photobox won hands down.
Their posters are also fine. I regularly use them as a cheap way of printing panoramas to a decent size by combining several on a single image.