Just when I was beginning to really like the G12, I stumbled across a problem while quickly trying various accessories with it.
One of the most viewed posts on this blog has been the list of compatible flashes that work with the G9. Actually, almost anything seems to work with the G9 although you do have to be careful about high voltage with older flash units.
I regularly use non-Canon flashes (I have Canon flashes also) because the G9 will sync at very high shutter speeds to non-Canon flashes. I've also used the Cactus wireless trigger and, more recently, the Yongnuo wireless triggers. The operating mode for high speed sync is manual exposure and manual flash. All these non-Canon accessories work fine on the G9. Unfortunately, they do not work on the G12.
Correction: My non-Canon flashes and accessories do "work" on the G12 but the G12 display screen blanks out as soon as it acquires focus (using half pressed shutter button). The screen doesn't really go blank, it shows the exposure without the effect of flash -- typically very dark. That is, the non-Canon flashes and accessories work but the G12 does not work very well. I may have a setting wrong but, if so, can't find it.
After email and telephone correspondence with Canon tech services, it appears that I'm the only one (first one?) to complain about the screen blanking problem when using non-Canon accessories (wireless and flash). Basically, the Canon position is that they are not responsible for performance with non-Canon accessories.
Although I understand the Canon position, the G3 and G9 actually work fine with the very same accessories. From what I've read, the G10 and G11 also work fine with non-Canon flash accessories. Using G series with non-Canon flash is a popular technique for getting high speed sync. I think this is a bug in the G12 firmware and will be corrected; however, the Canon emails and representative gave no hope of a firmware update. Therefore, after waiting a few more days while hoping for an encouraging email, I'll be returning the G12.
Too bad, I was really beginning to like it.