I'd like to put in a word in favour of the Polar heart test, or at least the one on my s610. (It's a test that uses the irregularity of your pulse when you're completely relaxed to calculate your maximum.)
It always gives me a consistent figure, and a couple of years ago, when I absolutely flogged myself in the final stages of a race, I got within a couple of beats of it for about 30 seconds, which suggests that it's reasonably acccurate.