I've done that the past couple of years two handfuls of times. Mostly to people I discovered on HN, with a nice hardware or software project. Only once did I get a nasty reply (probably because I was too much in my enthusiasm), so I'll remember you, you Italian prick. ;)

Most replies are very nice. People really feel seen and appreciated when I compliment them on their cool project and nice write up.

Even if no one replies, I would still send the occasional email, because I want independent websites/blogs to thrive and stay.