I own both a 305 and a 405cx. I use them for running and biking. My suggestions pertain to the 405cx.
1) Please please please please make it easier to change sports. It's a serious pain in the ass having to click and scroll through so many menus with the bezel just to change from running to biking. Surely Garmin can come up with a quick solution (e.g. Push and hold "start" and "menu" on the bezel. Pop up menu for change sport comes up.)
2) Data fields - I would like the option to display more accurate information in data fields if the Training Screen has only 1 or 2 data fields.
Example:
Training 3 has 3 data fields: time of day, HR, and Calories. Not much space, so time of day is HH:MM, HR is a single number, calories is a single number.
Change Training 3 to 1 data field: time of day. Time of day now shoes HH:MM:SS because it has much more space.
This is a simplistic example, but one that I would personally find useful.
Thanks.