I think it is very useful and have used DOF preview since I bought my OM4-Ti in 1990. Of course, OM lenses, and many others have had that feature for far longer than that.
+1
For me DOF preview is highly important and that's why i avoid cameras that don't have DOF preview.
As for the thread topic, for me these are the RF advantages:
- Smaller size (for fixed lens rangefinders)
- For interchangeable lens RF, wideangle lenses can be made much smaller due to no need for a retrofocus design
- no viewfinder blackout
- sometimes there is lower shutter lag
- usually they're quieter
- some RF cameras can be focused quickly
Disadvantages, in my opinion, compared to a SLR:
#1 - no way to see the actual perspective compression effects / DOF compression effects the lens is making
#2 - Lenses longer than 135mm can't be focused reliably or they don't exist (unless we consider those horrible SLR-conversion contraptions like the Visoflex, which turns the camera into a mutant SLR)
#3 - need auxiliary viewfinder for focal lengths that are outside the standard 35/50/90, or for all of them (depending on camera model)
--- which mean you need to focus through one viewfinder and then compose through another
--- which mean you need to correct for parallax with long lenses
- no zoom lenses available (and no, the Tri-Elmar is not really a zoom)
other disadvantages:
- wideangle lenses of non-retrofocus design usually show vignetting at wideopen, and since the late 60s retrofocus (SLR) wideangle lenses have been made as good as they get. Also, they are invariably slower (although smaller, so this is not really a disadvantage.)
- lack of "mirror shock" is not a real-life advantage when comparing to the more refined SLRs like the Leicaflex, Canon F-1, Nikon F3(and later models), etc. Even the Canon A-series (AE-1 etc) are very very soft in this regard.
- screwmount (m39) rangefinder lenses need to be screwed in carefully so you don't hit/kick the rangefinder coupling lever (it is sensitive)
- rangefinder assembly is sensitive to shock and misalignment; you need to be gentle
- with a 100% viewfinder SLR (Nikon F, F2, F3...), you can frame exactly. Not all RFs are that precise with regard to framing.
- usually fewer metering options