I have a Garmin Vivoactive. I love it, and as an insomniac I was interested in the sleep stuff, but I wouldn’t get your hopes up on that. The sleep tracking is not particularly sophisticated and as Sattua says, it can easily mistake stillness for sleep. I don’t think it can identify REM sleep, either. Having said that, it might not be a bad prelude to convincing a doctor you had issues. My husband and I both have one, and although he logs fewer sleep hours total than I do, he gets about twice the “deep sleep” hours. But that’s not telling me anything I didn’t already know, so other than confirming my beliefs I’m not sure it’s all that helpful.
What I really like is the GPS for running and hiking. It gives a much greater degree of accuracy on distance (and therefore pace) than something like the Nike sensor I used to use. Plus it is cool to be able to see your route!
Last week I did a hike I was unfamiliar with - it was my understanding that I’d eventually come to a waterfall, but I hiked and hiked and never did. I wanted to be a bit conservative since I was by myself, so I said “okay, I’ll hike for 3 miles, then turn around.” Thanks to the Vivoactive, at all times I knew how far I had already hiked.
When I got home, I was able to see where I’d gone and then look at Google Earth to figure out why I never made it to the waterfall. (You have to veer left at one point where I kept going straight.) I loved being able to do that.