We tested the Luxi Incident Meter app against a real professional grade Meter in the studio. The results were somewhat surprising. The Luxi attachment for Iphone worked well as long as the fit was secure, however I did experience problems with light leaking in. When it worked the reading was the same on the Luxi as it was with the real light meter. When light leaked in from the side of the Luxi, the reading was off by two or three f/stops.
All in all the app is worth looking into, especially if you mess around with it and get it to fit snug. The reflective metering mode was exactly on par with the professional device so I would say for a free app, it was well worth downloading.