A camera is a camera is a camera. If your aim is simply to take photographs, then continuing to use what you have is the most logical choice. Changing your equipment usually doesn't have much of an effect on the the pictures you take, and your photos should always take priority over your gear. If your gear is holding you back in some way, then by all means get something else, but it's hard to see what a Leica can offer you that your Nikon can't.
I am more of a gear-head than I am a photographer, and as such, I have acquired a hell of a lot of cameras. I have done this more out of mechanical curiosity than out of desiring better gear to take better pictures. In the end, I have found that one camera is much like another, regardless of the brand, type, or price. When I look at photos I have taken with Nikons, Canons, Olympuses, Leicas, Contaxes, Pentaxes, Yashicas, and others too numerous to name, I find that in most cases few if any would be able to tell what kind of camera took which. That being the case, what logic is there in paying $4000 for a later-model Leica with a decent lens when a $100 garden-variety camera and lens will take a picture 99.5% as good?
When comparing a Leica M to other types of cameras, there are a few things which I like. First, the smoothness of the mechanism, and second, the quietness of the shutter. This varies from camera to camera, depending on it's age, and how it was last serviced. No two Leicas feel or sound the exactly the same. Next, I like the rangefinder itself. Compared to other interchangeable-lens rangefinders like Contax, Nikon, Canon, or Konica (all of which I also have), it is cleaner, clearer, and more precise. Lastly, the ergonomics are excellent; the shutter button and winder are where they should be, and the camera seems to fit in the hand like nothing else. The lenses are excellent, well-made, smooth-focusing, and can deliver excellent images.
The only thing I really don't like about Leica cameras (and lenses) is their price. A Leica costs several to many times more than other cameras, yet the resulting images are only fractionally better, and that's only to those who know what they are looking for. An average person would never be able to tell the difference between the photos taken with an MP and a 50mm Summilux and a Canon AE-1 with an FD 50/1.4 (which can be found for the price of a Leica soft-release button if one looks hard enough).
If you want a Leica, and can afford it, then that's all the reason you need to get one. I don't shoot Leicas because they make better photographs than other types of cameras, I use them because they are a pleasure to shoot.
Man that is so much complex to read. My idea is simple. Before I think about purchasing something. I ask "Do I really need it". Then, before I commit, I ask "Does it really fits my style". Just don't buy anything without testing it out thorough for few days.
