I am sure everyone that makes espresso at home probably has that one useless accessory that is just taking up valuable kitchen space or more importantly was just a waste of money. For me that one accessory without a question has to be the tamping mat.
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When I first got my espresso gear I picked a steam pitcher, thermometer, tamper and the tamping mat. I got the tamping mat because I thought the tips of the portafilter would gouge a hole the size of Texas into the counters of my rented apartment, and I could just see the deductions coming out of my deposit with every tamp of the portafilter I made. However the portafilter spouts are anything, but sharp.
After a week of use I think it has remained largely leaning up against my espresso machine just sitting there. You may be thinking to yourself, well I may still want one so I can always ensure I have a clean place to tamp and not get coffee grounds on my spouts. If the mat that you get is just black rubber like mine it seems that the coffee grounds stick to it a lot easier than the granite or wood counter tops that I have had experience with. It's also just another item to clean as oppose to just wiping down the counter after making espresso.
One other item I believe led me to getting the tamping mat was that it's in coffee shops! However, to me it makes sense that they would want a tamping mat there, because they very well may get the gouge representation of Texas on their counters since they are making many 100s of drinks a day. So even the dulled edges of the portafilter probably would dig in.
So in my humble opinion, if you are thinking of getting a tamping mat, just hold out and get something you might like a little better.