Ask HN: How concerned should we be about USB security?

I had an ISP tech come by and set up service at my house, and I needed to access my router over Ethernet. My laptop doesn't have an Ethernet port, so I borrowed his Ethernet/USB dongle, I got everything set up and called it a day.

But I've started getting a little concerned about using this untrusted dongle on my laptop, especially from a internet service tech who may or may not be plugging his dongle into all manner of devices around town.

How concerned should I be about this? Should I trash my laptop and any accessories I've plugged into it since? This device is my central point of failure, I log into my banking accounts, admin accounts, it's my journaling medium -- you get the idea.