I have been using Garmin Connect modern for the last few months and have the following 2 problems/feedback items which make the new version unusable at the moment:
The major one is when using the "old" Garmin connect website on Linux (Mint 16) I would click on the Upload option, Garmin Communicator would then find either my Forerunner 210 or Fenix, and I would be given the option to upload all new activities. On the modern version of Garmin Connect, this option to upload from a device is missing - you can only save a manual activity. The automatic sync option does not work on Linux like it does on Windows and presumably MacOS. Please allow manual connection to my Garmin devices to upload new activities.
Secondly, after syncing my new activities on Windows 7 or Windows 8, they do not show up under Reports, whether viewing in a small window or opening up the reports page. The last activity synced is always missing - eventually it becomes available, usually the next day when I check.
Other than these 2 hiccoughs which can be fixed, I love the new interface.