I'm another one that has an R-D1 and loves it but I keep considering getting an M8. What I think:
M8 Pros
- wider viewfinder allows using lens as wide as 21mm (full vf area) without external vf (which is a hassle)
- longer rf baselength makes focusing more precise
- framelines are parallel to the viewfinder, not skewed...
- lower crop factor allows closer to original fov
- 1/8000 allows shooting wide open on daylight, interesting with very fast lenses
- bigger LCD and zoom wheel allow slightly more accurate focus check
- LR3 processes high iso files much better, making them more useable
- feels more like an M
- speed wheel is softer, shooting manual gets easier
- lightmeter in the viewfinder is more intuitive for those used to an M6/7
- reads SD cards bigger than 2GB
- shutter sound is less metalic than the R-D1's
M8 cons
- strong IR issue, need to invest on IR filters for each lens if shooting color
- some lenses just won't fit, like the DR Summicron
- noisier high iso than the R-D1's
- almost 2x the price of an R-D1
- SD card and battery loading is a bit clumsy
R-D1 pros
- since it's a rare and odd camera, nobody thinks it's a digital, so it can attract less attention sometimes
- folding LCD helps prevent chimping (but a halfcase on an M8 would do so too)
- manual shutter cock makes it slightly more silent than the M8
- side door for loading the SD card is faster than the M8's bottom plate method
- 1:1 viewfinder allows shooting with both eyes open, a very interesting way to frame
- completely analog feel, none of the basic settings require the LCD to be adjusted, which is great
- if something simple malfunctions, it might be possible to fix at home, although a bit risky
R-D1 cons
- 35mm lines are not so good to see for glasses wearers, lack of bottom line for 35mm doesn't help on that
- skewed framelines
- 45mm is the widest equivalent focal length on the viewfinder
- max 2gb cards
- shooting wide open at 1/2000 and iso200 on daylight will overexpose and get highlight clipping (but you can always add an ND filter) ps: yes, it might be a silly con, but still..
- heavier than the M8 but doesn't feel as robuts/solid
- not so much more silent than the M8 and the shutter has a metallic sound
- won't need an IR filter for most of the shots, but some synthetic blacks will come out purple-ish, as well as ember and orange heating lights
- crop factor tightens the fov of lenses: not so nice for wide angle lovers
- if something malfunctions, it's harder to find a technician that will fix it depending on where you live
i think both have strong pros as well as strong cons. it's more a matter of which will please you or annoy you more.
