I've used them, but they aren't the same finish/fit as the Stihl. The pin is way heavier then the Stihl version which doesn't help acceleration, and the rings on the last tecomec I used were 1.5mm not the 1.2mm used by Stihl.
The 026 pistons sold on EBay by lsquintos are actually new Stihl pistons, for a fraction of the price of new. I've used plenty of these...