I am not one for buying gadgets really. This is partly down to having a low-income and partly because I find a lot of them gimmicky and suspect that people buy them and then they soon become outdated when a new and improved version comes out. What have you wasted money on and then wished you hadn't?
No, I don't buy those "useless" gadgets like smartwatches, I only buy the things that can be of use to me like cellphones or smartphones. Although I wish I didn't buy as many spycams when I was collecting them, because I don't really use them. The money can be used to buy a tablet or something else.
I'm not fond of gadgets though I did purchase one several years ago that I wish I didn't. It was a camera that prints pictures instantly (like the Polaroid) but this one prints it on a sticker paper. I only got to use it once and up to now I have no clue as to why I bought that gadget in the first place.
My most useless gadget is probably this sellotape dispenser, I find that it is very useful, as it saves you the problem of having to find the end of the sellotape, but the main problem is that I hardly ever need to use sellotape, and so therefore it is just sat collecting dust.
Smartwatches are pretty useless but I don't really buy into those. But I did bought one gadget in particular, okay it's more of a addon, the Kinect. When I bought my Xbox One it came with the Kinect which for the life of me I never found any real good use for except for novelty purposes. I also bought a laptop cooler which has done anything but cool my laptop.