I have not used any 3d Printing services however I am an owner of one (Christmas Present to myself).
The quality of a print is down to what setting they the printer is run at. Smaller layer heights etc means it takes longer, however can print much finer detail. I printed my Dad a camp fire (less than 1cm x 1cm which took the same time as a pig style apron 4cm x 4cm due to settings and extruder speed.
I think as for advice, I would be asking what print setting they are using for their prints. You will always get the lines from the extruder moving along the X and Y axis however as my Dad has seen this can be minimised by the settings (which takes longer).
I hope this helps you out.
I do love what you can achieve with the correct software, knowledge and printer.