Is anyone loving their 645?

I completely understand the frustration and regret from people with different expectations.

The Fenix 5S Plus could have been a great watch, but the strap is too short and narrow to support the bulk of the watch when you are running and sweaty. I think the 645 feels much more comfortable to wear because the lugs don't stick out, so it wraps around your wrist better.

I have a 645 (no music) and I'm very impressed.  I'm a hobby jogger and data scientist by trade.  I run 6-8 times a week and my runs are typically  4 miles to 15 miles at paces normally between 5:40min/mi to 8:10min/mi.  I run mostly in New York City, unfortunately find myself running up Madison Avenue amidst very tall buildings to get to Central Park or cross town on 60th St. to get to my three favorite bridges (Queensborough, Kazimierz Pulaski, Williamsburg)  My 645 GPS is consistently locked in and accurate.  My running partner (who likely has never upgraded/synched his driver via the easy-to-use Garmin Express application) shows me ridiculous and inaccurate plots of our runs amidst the buildings using a much fancier and significantly more expensive Garmin watch.  The Optical Heart Rate feature on my 645 is also eerily responsive and reasonable.  My heart rate max is around where I should be for my age, I only hit it after hammering Harlem Hill repeatedly at maximum speed.  The heart rate falls off quickly when I rest during intervals and responds independently of my cadence.  (My running cadence is typically 178-180 and my moderate effort HR is 145-151).  So far, these metrics are uncorrelated.  Everyone told me buy a strap as wrist-based optical heart rate is not to be depended on.  This 645 heart rate feature using the 4.20 drivers operates perfectly fine for my threshold-based and tempo training.  I might hypothesize Garmin allocated their ace programmers on the core functionality (GPS/HR) watch drivers in the past two years because the software quality is surprisingly good compared to all products I've ever used.  Additional disclaimer, I am 5'11', 144lbs and I can literally see my pulse by looking with my eyes at my wrist.  These characteristics might simplify the software challenges.  All in all, I'm somewhere between pleasantly surprised and super impressed for my use case.  (Running recreationally in the great city of New York)

No.

It was fine, but lately I get altitude gains/losses of 30k ft on a flat run.

Plus Garmin won't update the 645.

I just wrote a 1-star review for this item on Amazon. After years of investing in garmin products to record my activities, they are really making me reconsider.

No device is perfect but I think that the 645M comes really close for me.  I wanted music (previously had Adidas Smart Run) and running data for race training and this exceeded expectations.  Running data is awesome.  Training plans were good via Garmin Connect on the desktop but are now at the next level with the interactive Coach on the Connect App on Android.  I experienced the body defect with the plastic housing cracking where the band pin is inserted but the US Customer Service support was excellent for me and I received a replacement immediately even though I was past the 1 year warranty period.   Refurbished unit is actually a bit better with stronger Bluetooth to the same headphones and seemly better battery life. Overall, I am loving my 645M and would still recommend it even with the other option out there.   

Haven't read all the responses but I upgraded with a VivoActive HR to a Forerunner 646M in March 2018. While there were some issues initially with the watch rebooting mid run due to music/audio alerts I haven't had any issues for many many months. I try to run 5-7 times a week in the morning with distances between 3.5km to 10km. I will often also do elliptical 5-7 times a week in the evening so while I am not a hard core runner or a marathon runner I do use my 645M a lot and wear it 24x7.

I just completed a 10k this morning and improved my personal best from 52 minutes to 46 minutes. A big factor in that improvement was the audio alerts on the 645M paired to my Beats wireless earphones. I had audio pace alerts playing every minute which helped me maintain a consistent pace around the 4:30 min/km mark.

Overall I am loving my 645M. Have no regrets on buying it. Would definitely recommend it. The only reason I might consider upgrading to the 945M would be the improved battery life and updated metrics. However I will likely stay with the 645M and then upgrade to a 655M (or whatever it is called) when it comes out.

The only 'issue' that affected me that took a long time to solve was the reliability of the bluetooth connectivity to my iPhone X/XS. It is much improved now although not perfect.

I would rate it 9/10 in meeting my personal needs.

TheSalamander