Comparing with my 5D.
Much larger everything. Including a battery that seems to last all day long. Love the huge RAW and JPEG files. The (M1) medium JPEG's are bigger than the 5D Large JPEG's. All that real estate makes cropping usable images better.
AF works well in poor lighting. One nice feature is the focus adjustment. I had dropped a 24-70 lens some time ago onto concrete - breaking the filter, but the lens seemed to be fine. after testing and adjusting the focus point - It turns out the lens was slightly out of focus, which the M3 took care of.
ISO button makes changes quick & easy. I also like the menu navigation better. Oh yea, love that auto sensor clean.
I'll put her through a season of weddings and I can already tell it's going to be great!
Added a 4G SD, 8GB & 16GB CF. It's also so handy to write RAW to the CF and JPEG's to the SD.
Live view - eh.
Very nice. No regret.