I bought a Series 0 last year in Space Black Stainless Steel to make if "feel like a real watch". I'm a watch guy and have around 8-10 watches mechanical, new and vintage - I have loved it ever since.
Even though I loved it, I still had some stuff there could be worked on.. The speed has been a major issue - I almost didn't use any 3rd party apps because of the speed and some of the few I used felt like they were running on a too slow chip.
GPS - I have been a runner most of my life and like to keep track of speed, distance, how much I rank up per month etc. The watch has been great for keeping track and non-workout-days the rings get pretty addictive to fill out. But I really like to be able to run without my phone - I don't want to be interrupted on my runs by texts, calls, mails etc. so the no-cell-connection is a win for me, but GPS chip a must...
Battery life seems to have been improved a lot as well in my real-world-tests... I often go to bed with 50% or above when I haven't been working out or 30-40 even with a workout... Pretty great stuff
All in all my upgrade to Series 2 couldn't have made me happier - I loved the look and feel of the Stainless Steel watches from the start and have different straps (thank god they can be used again) but speed, GPS, waterproofing and a tad brighter display just makes this thing a winner and a far more serious tracker for fitness - love it.