I am thinking about and would appreciate feedback on upgrading to a Garmin 500 or 510. Currently on a couple of bikes I use the comparatively modest Cateye Strada Wireless without cadence for speed, mileage etc and thinking about replacing one with a Garmin.

So a couple of questions please:

If I don't feel a compulsion to spring for a power meter, is a Garmin really worthwhile?...or is a Garmin really only worthwhile as a basis to monitor power output while riding?

Cadence isn't that big a deal...I tend to spin versus mash anyway so this wouldn't be a big help to me..

But how about heart rate? Do you guys without a power meter use the heart rate monitor function on your Garmin to train within particular zones? Is this a good reason to get a Garmin if no plan for a power meter?

Lastly...the display for aging eyes. Can font size be increased on the display such that you only show maybe, speed and heartrate on one screen with a larger font?
Thanks