I was saving for an M6, but when this M3 popped up for a good price, I jumped on it. I've always wanted to see what the allure of the Leica was about, and since I couldn't find one to borrow or rent, this is what happened. The viewfinder is gorgeous, the build quality amazing, and it feels better than any of the Japanese rangefinders I've used.
The thing with Leicas (and other rangefinders such as Voigtlander Bessa models) is that you need to get the one that offers the bright lines (the viewfinder options) for the lenses you intend using. The M3 is optimised for 35mm, 50mm, 90mm and 135mm lenses. You can use wider lenses, but you'd need an external viewfinder that would sit in the accessory shoe.
I've owned an M6 and an M7 and both are excellent cameras. However, the M3 (and the M2) have (in most Leica users' opinions) the best build quality, along with the MP. As Francois mentioned, there can be an issue with burning holes in the shutter curtain but, as far as I'm aware, that only happens with prolonged, direct exposure. I've never had any problem shooting into the sun but I tend to keep the front of the camera covered / the lens facing the ground while I'm walking around with it.
Have fun, use it lots and post the results
