I have the Cygolite Expilion 350, actually 2 of them. The Expilion is well made, relatively small, light, has excellent mount. The light is more than adequate for commuting. It also has a removable battery and you can buy a spare one from Cygolite.

The reason I got 2 is I am a believer in having backups... if one conks out I still have another to fall back to. Bonus is I mostly use them in medium mode which means longer run times. I have been getting around 4 hours run time at medium before the low battery warning indicator starts flashing. Admittedly this is shorter than the advertised 6 hours at medium but I have been using the lights in sub zero (celcius) conditions.

Here is an image of the 2 Expilions at medium mode (the 3rd brightest out of 4 steady modes) compared to the light produced by a streetcar: