I agree, Claire, 35mm rangefinders have a clear set of limitations, and for many applications an SLR is really the better, or at least more flexible, choice. In the past, Leica produced a number of interesting innovations, such as the Visoflex, in an effort to overcome the inherent limitations of rangefinder design. But, some of those innovations now seem a bit Rube Goldberg-ish. I use Dead Link Removed on my Ms that has a (permanently-attached) set of "eyes" to magnify the view with the 135mm framelines. Unfortunately, this lens is rather large and heavy (at least by Leica standards), and is no longer manufactured. But, it's an excellent lens and the shallow DOF at 2.8 (at which it is still very sharp) is great to work with. Dead Link Removed.
At the other end of the spectrum, macro, rangefinders are again limited. The older solution from Leica was Dead Link Removed that shifts to a closer focusing range with the addition of the "eyes". But, the DR only focuses to 17" or 19", depending on the model. Absent the eyes, though, the DR Summicron is still one of the sharpest 50mm lenses available, and has a wonderful signature. Dead Link Removed in normal range, taken on a foggy morning at the old graveyard in Natchez, and Dead Link Removed. Here's Dead Link Removed that shows its sharpness at more conventional distance. Although I'm not particularly good at Dead Link Removed, that and candids are where rangefinders are probably at their best due to size, quiet operation and general stealth.
At least with Leica, and probably Zeiss rangefinder lenses, too, another distinction is that they perform very well wide open, whereas many 35mm SLR lenses really need to be stopped down at least a couple of stops before they perform well. Plus, the light weight of a rangefinder tends to allow hand-held shots at considerably lower shutter speeds. Dead Link Removed, for example, was done at 1/15 at f/2.8 on FP4+ inside an old battleship.
Each person, I believe, needs to make their own decision based on their personal style, what they shoot, and what they are looking for. I like the Leica for its small size, light weight and stealth - but mostly for the distinctive signature of Leica lenses. But, I still use my Nikon SLRs when I want auto-focus, flash sync at higher speeds (the M only syncs at 1/50 or below), real macro, or Dead Link Removed. The Leica M also tags along when I'm out with the 4x5 or 8x10, as its light weight and small size allows me to document other elements of the LF shoot or locale conveniently.