I use the LightMeter app periodically. Aside from annoyances with the interface, it works well enough for me. If I'm shooting seriously, I usually use a dedicated meter. iPhone metering is more of a 'verifying what I think the light is' tool when I'm shooting casually.
Many of the apps over complicate it. Others work, but have more complex interfaces. My only complaint about LightMeter is that it doens't seem to save the 'locked' setting. I want to set the iso to something specific and have it remember. And I want it to remember which setting I'm calculating.
Other apps seemed like they were either too complicated or more buggy, though I'm sure there are other good ones out there.