Flat.  Convex.  Concave.  Rippled.  Aluminum.  Stainless.  Pressure calibrated.  Does it really matter?

Do you really need to spend $100 on a tamper?

My personal opinion is no.  I’ve tried tampers that are flat, convex, and rippled and I couldn’t taste any difference in the result.  They all have their theories as to why non-flat is better.  All the theories have something to do with water dispersion on the puck.  But, in reality, the shower screen of your machine is doing that job.  And, when the puck is wet, it expands to fill the portafilter all the way up to the shower screen.  So, any shape that was in the dry puck is not going to stay.

The most important aspect of the tamper is simply the size.  Make sure it’s the correct diameter for your portafilter.  If it’s not the right size, it can be shaped like your Aunt Martha’s face and it won’t matter.

You can find several of the tampers I’ve tried to the right there.  The exception is the Espro calibrated tamper.  I just can’t bring myself to pay $100 for a device that flattens coffee.  I watch videos of the best barristas in the world, and they use simple flat tampers.  So, if that works for them, I can certainly spend the time to learn proper technique.