ATTQOTD: I am a huge gadget geek person in all things, so buying gadgets/accessories for running is right up my alley. Thankfully most of them have worked out pretty well. Some gadgets that I use regularly and love, other than the obvious Garmin, are my Jaybird wireless earbuds, my Scosche Rhythm+ armband HRM, and now my Tracer360 glow in the dark vest (once I managed to figure out how to adjust the straps so that it didn't keep falling down my shoulders). I even wore it on Christmas Eve morning for our group run.

It was a gray and gloomy day, and I was wearing a Santa hat, so I also put on the vest and set it to the green setting.

In the shoe and clothing departments, my experiments with Altra shoes and Sparkle Skirts have also been great, love them both. In the fuel department, Huma gels were also a good experiment, love them.
The only one that I can think of that did not work out off the top of my head is my Nightrunner 270 shoe lights. I got them for a Christmas gift last year from my SO, but did not really do any dark running over the winter/spring. I mainly wanted them for either dark running in the summer, which I didn't end up doing, or dark running in the later fall/early winter, so I never actually used them until that first run I did with my new Wednesday night running group the day before Thanksgiving. Well, one of them didn't turn on! And they weren't really as illuminating as I had hoped, so I was glad to also have a headlamp on. I only had the one that was on, but the light throw was not really very far ahead of me, although maybe it just needed to be adjusted a little. I think it's still not as good as a headlamp though. I think it's probably too late at this point to try and get a replacement for the one that doesn't even work since it's been a year now. Womp womp.
Edit: Oh, reading the other posts, I'll add Race Dots to the list, kinda sorta... I'm on the fence.

The magnetic bib holders. I do really like mine, but they are difficult to get put on. And one of mine rusted, so I need to get a new set because it keeps leaving marks on my shirts.