The Mamiya 6 is one of my favorite cameras. I have two bodies and use them constantly for all my work, much landscape as well. First of all I would not worry about framing a distant landscape wrong, you simply crop if you need to. I find the framelines close enough for my work. If you're one of those people who think cropping is absolutely a no-no then do not ever get a rangefinder!
As Ralph said, the optics are amazingly sharp. I believe the sharpest optics in MF. But some say the bokeh is not flattering. RF's aren't the best choice for head and shoulders portraits, environmental portraiture absolutely, but not in tight. And you can forget about close up photography. There's also no super long tele, though I find the 150 so sharp that I can crop my negatives a great deal and still get wonderful results. I find focusing with this system easy and fast. With the 50mm I will just use the scale on the lens. The lenses are slow though, f/4 at the fastest.
All that said, this system fits my style perfectly. I prefer to work quicker and lighter and this system allows me to do so and still shoot MF. And when I want to slow things down I just put the camera on a tripod, which I try to do for most shots, but to be able to use it so easily handheld is a huge benefit.
I honestly wouldn't worry about the parts thing. I've had two bodies for years, used them hard, and make sure I have Bob at Precision Camera works CLA them every once in a while and I'm good to go. They feel great!!
If you have any questions about this system feel free to PM me. I have lots of experience with this camera and highly recommend it.
Here's a brief pros and cons list:
Pros:
Easiest MF to shoot handheld (because it's square)
Lightweight
Small for MF
Sharp Optics
Fast to use and focus
Quiet
Easy to focus in very low light
VF doesn't black out at moment of exposure
Filters can be used and not obstruct the view through the VF
Cons:
Slow Lenses (A con for some people)
Lenses do not focus close
Only 3 lenses in the system (again, a con for some people)
No really long tele
Meter is not the greatest. I use a handheld meter most times
Cannot see TTL (this doesn't bother me either)
Some filters are hard to use (Grad ND filters)