What about the Sigma 18-50mm f2.8? Is the difference between f1.4 and f2.8 great enough that I should stick with the 30mm f1.4?
Have you ever used a lens as fast as f1.4?
Depth of field in incredibly thin. If AF is just a little off, you are out of focus. Even if you hit the AF right on, it can still look soft. f1.4 lenses are not typically good performers at f1.4 because most of those lenses are old designs going back 10-15 years or even more. The more modern designs like the Panny/Leica 25mm f1.4 and Sigma 30 and 50mm 1.4's will be better f1.4lenses.
While newer,and the Sigma 30mm f1.4 can be very good, I've alsoseen plenty of complaints from Olympus DSLR users about it being soft at f1.4, some of which may be users not used to using a lens that fast, and some could be Sigma's well known quality control issues with many lenses. If you buy a Sigma lens and don't think it's producing what it should, send it back and get another one. It alsomakes sense in buying Sigma lenses to make sure you are buying from a store with a consumer-oriented return policy, not just the store with the lowest price.
An f1.4 lens doesmake the finder extremely bright and easy to use all the time since viewing with areflex system is done wide open, but you will be better off most of the time using an f1.4 lens at f2, saving f1.4 as a last option.