The Vivoactive is the best watch I've ever owned! I logged 5400+ miles last year with it and uploaded via Bluetooth to Garmin Connect/Strava-- definitely have put it to the test in terms of usefulness/durability/accuracy! The battery in GPS mode lasts 7-8 hrs, which has gotten me through 50 mile/100K races. I haven't seen any decline in the longevity of the battery with exceptional use.
I had a Garmin Forerunner (405?) at one time, and it did my head in. Too much bells and whistles and took forever to set up for a workout. Plus, it would hit my wrist bone-- the Vivoactive is thin/lightweight and finally (maybe because of the square design?) doesn't hit the wrist bone at all. It's much simpler to use and setup (~for timed intervals)-- a single start/stop button and has whatever apps and widgets you want on it. The only thing that was kinda weird at first is it alerts you via vibration and doesn't have an audible beeping alarm.
There's an app store-- I specifically needed an Interval Timer (the one I use is called My Timer; I also have another called Timer). You can set it up for a interval time+rest interval. I'll do like 6 min interval/3 min rest and set it up to repeat (vibrates to alert you between intervals). I'm not sure you can set it up for measured distance intervals or to know the pace of your intervals (which I personally don't need because I train and race by effort). If you had it in Run mode you could tap it to record splits.
https://apps.garmin.com/en-US/devices/vivoactive/appTypes/application/apps
Of course, the other great features are that it's a Smart watch and activity monitor. You can set up whatever notifications you want to receive.
Let me know if you want to know anything else. It's been a great watch and probably the most bang for the buck of any Garmins.