I spent a couple of years shooting my Oly "35" rangefinders exclusively, just to give them an honest trial. My observations:
- An SLR will do anything a rangefinder will, but not vice-versa (marcos, tele, etc)
- At least for Oly gear, the shutter noise difference is negligible
- Except for the 35RC and XA's they are no smaller than an OM with equivalent lens
- The focus on a RF can be faster, but my eyesight is not the best (astigmatism). With good eyesight and a split prism focuser I believe the SLR can be focused as fast.
- The camera shake problem is much worse with the rangefinders. This seems counterintuitive, but with the rangefinders you are setting the exposure with the shutter button. This requires a very long, stiff shutter button to trap the needle and set the aperture, and you were never sure where in the travel the shutter would acually release. I almost never get camera shake with my OM's even down to 1/4s, but with the rangefinders shake was not uncommon at 1/60's. (This is not a problem with XA's, but that little shutter button has its own problems.) If you are shooting print film, of course, you do not need a meter and can use an older RF without built in metering. But for slides, you need an external meter which I never found practical. (It is somewhat different with a MF folder, where the metering is just another step in the overall creation process. There I just accept that about 1/3 of the shots will get tossed for bad exposure.)

So while the RF's look lovely in their display case, the OM's go with me now when there are pictures to make.