I use it exclusively. I have, on one occasion, found it to have gone bonkers, totally giving me whacky readings, which I was fortunate enough to catch (thanks, sunny 16 rule!). A reboot of the iPhone fixed it up.
It's great to be able to have a visual feedback of exactly *what* you're metering for... something that is not easy to get unless you have a good, and expensive spot meter. Highly recommended.