It may sound strange, but the biggest advantage that I discovered, is using a very manual camera, the simplicity of it. There is no computer/software to get in the way. When I'm out shooting, I don't spend any time digging through the menus on the LCD screen. The camera does what I tell it to do, so as long as I'm mindful about what I'm shooting, the pictures will come out ok. That's the trick, I think, to be constantly thinking about the next shot. It's like shooting a non-automatic gun, you'll never forget to cock the trigger hammer because you'll incorporated it into the process after a bit of practice, almost like muscle memory.
The biggest surprise, to me, has been the joy of reviewing a day's captures in the evening. The LCD on the back of the M9 is poor, to the point where I think it's only there so you can change settings in the menu. You can use it to review images, of course, but since the LCD output is so poor that you should never discard an image purely based on what's shown on the LCD. Nowadays, people want the instant gratification of seeing their images on a high resolution LCD screen, and even "live view", so they can see the image before it is captured. I've had it on the Sony NEX camera, but I must say the delayed gratification is even more rewarding when I load the images into my computer at night and really see them for the first time. It's really counter intuitive, I know, but I'm constantly wow'd by the color and rendering of the images. Now seeing live-view, to me, is like getting the spoilers before you had a chance to watch the movie.
I'm also surprised by the rate of keepers I'm getting. Going back to the gun analogy... with a fully automatic weapon, one tends to just hold the trigger and blast away and hope it hit something. But since I'm giving more forethought when shooting the M9 and taking the time to aim (focus), I'm more likely to get what I wanted. I suspect that's why even though I've just recently acquired the Olympus OM-D camera, I'm still reaching for my M9 more. This has quenched my GAS ("Gear Acquisition Syndrome") thirst considerably because I'm simply enjoying shooting with the camera I have and getting great results from it.
The Leica M9 still inspires me to take more photos everyday, and I look forward to the coming summer months and hopefully I'll be able to go out more and capture more images to share!