IMHO...a power meter is an indispensable tool if you are training for competition. If you aren't, it's an expensive toy (even used). You can get very useful metrics from a heart rate monitor, and if you use Strava, the power estimations it provides are fairly close. As for functional threshold power, it is individual. I've learned that there are those with silly high FTP numbers who can't sprint, and those with low FTP's who regularly do well in road races and crits.
Me? I have a power meter (PowerTap), because I have a coach and I race. My FTP is relatively low, even compared with other women my age. However, I get along quite well, thank you. Oh, and I'm over 60!
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