I have a 300 lumen Cygolite Metro and it is adequate for commuting (and, in my experience, outshines 90% of lights that I see at double centuries). It is plenty bright, but I wouldn't mind having extra lumens in the form of a wider beam.

There are two drawbacks of going above 300. Standard headlight design consists of a LED bulb and a rechargeable lithium battery. Good LEDs don't differ much from one another and give to the tune of 60 lumen per watt. Lithium batteries are 120-150 Wh/kg. That means that a 300 lumen headlight with 3 hour life will require a 15 Wh battery that weighs at least 100 g. (And then there's the LED itself, casing, lens, etc.) Most lights are charged through a USB port, which, by design, can't deliver more than 2.5 W, and it would take 6 hours to charge a fully depleted light.

If you go from 300 to 600 at 3 hour life, that gets you to 200 g and 12 hours to charge. Any further, and you end up with a light that's heavy as a brick and requires a dedicated charger because USB does not cut it any more.