There are
many reasons why I prefer rangefinders to SLRs, and not just for street stuff (which I seldom do anyway). I use RFs for everything from 35mm to 4x5" and I love 'em!
Foremost, composition with a RF isn't done through the lens, and as such you have a 3D view of the subject. You have a big, bright viewfinder and there is no blackout when you fire the shutter. Some RFs have 100% viewfinders so you can compose with both eyes open; I usually keep both eyes open even with non-100% VFs, I feel more comfortable doing that because the VF is so bright and, again, the view is 3D. I can't keep both eyes open with an SLR, it is just too uncomfortable.
Also, the framelines in most RFs shown space
around the composition, which is a feature that I find very useful. I like to frame the shot just as I would with my hands. With SLRs I feel more cramped when I compose. The RF style of composition does require that you develop some knowledge and intuition about depth of field... I focus hyperfocally. There is no DOF preview.
Also, concerning composition, I am an avid fan of infrared photography, which of course requires an opaque filter. With an RF, you can compose normally, you aren't looking TTL so it doesn't matter what filter you have on. I also tend to use red and orange filters so again I feel like
not looking through the lens is a bonus.
Now, concerning mirror slap, that is of course the most commonly cited benefit of RFs. With good technique you can quite easily work down to 1/15 sec handheld, sometimes further. The speeds I tend to like best are ~1/60 and slower, I am musically inclined so perhaps those intervals are just more familiar to me and I can compose the shot with specific rhythm in mind. Some slight motion blur can be, I think, a very powerful compositional element, I am not such a huge fan of static scenes, so I like slightly longish exposures for mnay things (albeit not sports or wildlife obviously). At 1/60 is right about where mirror slap starts to enter into SLR shots, so unless you have IS/VR lenses then you can't really go down to 1/60 and beyond, handheld. And those lenses are expensive and bulky as heck. I have done handheld shots as low as 1/8 sec with an RF that I felt were plenty sharp.
Concerning optics, there are some almost miraculous RF lenses that have quite simple designs because the lens sits so close to the film plane, so they don't require nearly the degree of correction that SLR lenses do. I won't go off into MTF technicals but suffice it to say that if you are interested in shooting normal to wide lenses, wide open, then a good RF is usually going to outperform a good SLR. I really don't think anything can touch some of the Leica and Carl Zeiss fast primes for RFs. With SLR glass one typically must stop down to f/5.6 or so to get away from lens issues, but with RFs, that "sweet range" is a stop or two wider at least. There are fast RF lenses that deliver excellent sharpness and contrast and lovely bokeh down to f/2, or even f/1.
That last point brings me to one of the real strong suits of RFs:
available light photography in general. Many of the issues I mentioned above combine to make RFs really ideal for available light. However, even if you use flash, RFs can shine. RFs that have leaf-shuttered lenses will synch at all speeds. The Leica M7, for example, can work with a metz flash down to 1/1000 sec!
Ergonomics is another very strong suit for RFs, they are typically light and compact. My mamiya 6, which shoots in 6x6 cm format, can go in my jacket pocket, even with the 50mm lens attached (it collapses into the body). I can take it amost anywhere and not feel burdened. In fact it has become one of my favourite landscape cameras. Now in terms of resolved detail, the medium format RFs can far exceed 35mm and can match e.g. a Nikon D2x; look at this:
http://www.diax.nl/pages/start_mamiya_nikon_uk.html
The mamiya RFs are really pushing toward large format's level of detail, and they are about as compact and ergonomic as a typical SLR at the same time. Much of that has to do with the lack of mirror box- think about some of the comparable 6x6 or 6x7 SLRs and how unwieldy they are by comparison, e.g. the hassies or the pentaxes.
Concerning shutter noise, well, my mamiya 6 is
very quiet, it is way more quiet than any SLR I have used. My quietest RF has to be the konica AF, that thing is scarily quiet. It is totally unintrusive. And just as I prefer available light because I find it less intrusive, sound and the physical bulk of the camera are important factors if you aim to get candid shots.
The drawbacks of RFs: you are usually focusing in the center and when you shift the focused subject ff center you have to think about whether the subject is still in your DOF. But handling that becomes second nature. Also, you can't really work past about 135mm, though some digital models can effectively take you past that now. And RFs are not hgh-fps instruments, you have to know your timing and know when to click. I actually see that as a benefit, I find the machine-gun approach wasteful and I think it really causes the photographer to become detached from the dynamics of the scene. But I am sure others feel differently

Some people are annoyed that RFs may meter through the viewfinder or from some meter that isn't looking through the lens. I think that's kinda silly, if you use a filter then you had better know the filter factor. It is really only an issue if you use a polarizer, that requires some thought
One other thing: you can get your mits on a good RF for much lower price than you may think. Don't be intimidated by those with the $5k kits, there are great outfits for
any budget. When I bought it, I think my mamiya 6 kit cost me maybe $1500 with two lenses, now it is worth more, but you can still get the body for around a grand. And there are many, many other options, like the ~$550 new bessas or the some of the older yashica gems that go for under $100 every now and then. My most recent RF purchase? A crown graphic 4x5 with a coupled kalart rangefinder, for ~$250! What a joy to use- handheld 4x5! I also use a $50 olympus XA now and then- now that's a true pocket camera! There is a rangefinder for everybody's budget.
Oh, sorry for the lengthy epistle. :rolleyes: