I've used Pocket Light Meter for the iPhone (like dpollitt :) when using my grandfather's old Leica IIIc, a 35mm rangefinder with no light meter or automatic metering. I played around with it quite a lot, comparing its results against those of my Canon 5DmkII's metering, and found it to be very accurate. The results from the Leica bore this out too: generally (where I hadn't messed something up :) they were well exposed. So that app at least is certainly useful and by no means a gimmick.

The same won't apply to all smartphone meters though. Without straying too far into Stackoverflow's territory, a smartphone light meter's effectiveness will depend on two things:

1. The quality of the phone's camera and firmware (i.e. does it expose well itself)
2. The means by which the software developers can access the exposure information

On Android phones, for example, the usual method is to get the camera to take a photo then read that photo's EXIF data to determine the exposure settings. However, not all manufacturers include EXIF data in their photos (and it's easy to imagine others - especially cheap, low-end phones - might include inaccurate EXIF data), so reliability will vary between models.