This is just MY opinion, and I'm in the minority, and I don't mean to imply that others are wrong -- clearly not, since most pro athletes use them.

I determined my max by hill running test. Around 206. The problem for me is I can run forever at what is theoretically around 85-87% max. And below 80% feels kind of pointless -- I tried really hard for the first 7-8 miles of a 12 mile run once to keep at 80, and it was incredibly frustrating -- I just gave up after a while. At nominally 85%, I wouldn't say my pace was easy, but my tongue was not hanging out and I wasn't getting fatigued. Since the hr max I arrived at is over 200, I doubt it's way low, so the percentages calculated from it are probably not too low either.... it just doesn't seem to work for me. Perhaps I'm not patient enough.

I would say it's worth a try though, since many people find it useful.