As a longtime Leica rangefinder (film) shooter, perhaps I can answer a few of your questions.
First, a bit of history. Leica of course goes way back. The concept of using 35mm film in a small, compact camera was a radical innovation in the 1930s. In 1954, the M3 was introduced, the first bayonet lens mount Leica, claimed by some to be the "best" Leica M. Through the late 50s it was THE photojournalists camera. With the introduction of the Nikon F in 1959... another major innovation, but in an SLR... the balance of marketing power gradually shifted from Germany to Japan, and from rangefinders to SLRs. However, the Leica M stayed in favor with many journalists and editorial and fashion photographers (for example, Jeanloup Sieff and Marc Riboud) at least through the late 1970s.
Since then, the Leica M has been largely a street photography and art camera... most famous for street photography because of its small size, near silent shutter, rapid framing ability, and discreet appearance. Many working pros use a Leica for their personal work. For me, it's my primary camera, but I no longer shoot pro, now I do gallery exhibits. Thus I don't need to worry about speed or long telephotos.
I can't speak to the Leica R system (the film SLRs) because I've never used them. The M8 is sweet (digital, 10MP), and uses M lenses with a 1.3 crop factor. The S system is not really out there yet, it sounds promising but thus far untested. It's the first real attempt by Leica to go after the top-end modern working pro. But note that in this market, folks are already spending tens of thousands on medium format digital backs, and depreciating their gear, so high cost is just part of the game.
What's special about Leica? Again this is based on the M-system rangefinders. But they are very special cameras, although not for everyone... nearly useless for shooting sports, for example.
They are solid, exquisitely balanced, minimalist... only seven moving controls on most models. Nothing to get in the way of the picture. Even the M8 has stayed true to that approach, with much simpler menus than most digital cameras.
Then there's the glass. Leica optics are legendary for good reason. While they're as sharp as anything made, sharper than the 35mm negative can handle in some cases... which only means that the lens is not your limiting factor, unlike with many cameras... it's really the less tangible things that set the lenses apart. There's a smooth tonal rendering, legendary bokeh (rendering of out of focus highlights in the background in a pleasing way), saturated colors, and just... the Leica look, which is impossible to describe in words. They're also among the very few lenses which perform well wide open, and of course one would expect no less of lenses often used in low available light. They work well with the M bodies, it's not hard to handhold and get sharp results down to 1/15th second or even slower, with nice balance and no SLR mirror slap vibration. Typically, I can get good results at two stops less light with my Leica than with my DSLR.
Looking through a quality rangefinder is a revelation, once you get used to it. You can actually see (gasp!) unlike through the tiny dim tunnel of the typical DSLR, which assume that everyone uses auto focus and thus viewfinders are an engineering afterthought. I'd expect quality viewing from Leica SLRs, although I haven't looked.
Cost? Yes, they're expensive. With functional models (excluding the collector pieces), you mostly get what you pay for. Quality build, reliability, high resale value. I paid $1700 US for a brand new M6TTL 0.85x nine years ago, and that same body today is worth, on the used market, just about what I paid for it. So all I've lost is inflation. I've been through two DSLRs during that time, and last year paid $1800 US for a Nikon D300, which is currently worth perhaps a bit more than half that much if I tried to sell it right now. In two more years it will be hopelessly obsolete and worth a fraction of the original cost.
Lenses are the real cost for Leicas. The common ones, 1980s or 1990s vintage 50mm and some 35mm lenses, can be found used for as "little" as $500-$800 US. But look for anything relatively new, or especially fast, or exotic (extreme wide angles, or the f/1.0 Noctilux) and it's going to cost you. But, I basically use two lenses, a 50 and a 35, both 20 years old, and rarely want more. The entire kit could fit in my jacket pockets, which is great for travel.
Hope that provides a little background. I'd expect the S series to be scary expensive, but then they're going after the relatively small top-end commercial market. If you're in a major city, you could perhaps rent one for the weekend once they're readily available. Go to the Leica website
http://us.leica-camera.com/home/ to keep up with the press releases.
