A friend recommended print24 to me and I have to say I am very thankful: it worked perfectly for me.
My order was 500 copies double-sided carton, full color, custom-size to be trimmed. All copies came out perfect, very high resolution in printing, no defaults (I checked every one of my 500 copies!). I particularly appreciated the many options available when ordering, which allow to customize one's printing product for professional quality.
My order was scheduled to be delivered 5 days later, on December 21st. The day after I ordered, I received an email by print24 saying that, due to the high volume of orders (Christmas time), most likely they would not be able to deliver my order on the 21st, but on the 22nd instead. I was a bit disappointed at first, but ended up receiving my prints on the 21st anyway! No reason to complain then...
Finally, their prices are truly competitive. I spent about half the money I was asked in the initial quotes I had from other online printing services, let alone high street ones...

In a word, flawless!