I got my Garmin because I got sick of my phone not recording what I was doing on Strava. To Jeff's point I found that Strava was reducing my enjoyment of the activities it tracked because it tells you how well (or not, oh so very much not) you are doing against other people. Instead of enjoying an hour on my bike I was agonising over how it was physically possible for someone to get up a 2 mile climb 5x faster than what I could manage. CLUE: I have two Olympic medalists living in my village.
The Garmin does sync with Strava but I don't look at Strava anymore.
There is a danger that your targets become a psychological rod for your back. Getting anxiety because you haven't done your 10,000 steps a day!
One thing I do notice with the metrics is that they can predict when I am going to get sick. It took me a while to work out that this was something that was being predicted. The only thing I worry about is where is this data stored and who might have access to it.
I know that bike thieves use Strava data to spot who is likely to have an expensive bike and where they live.