Very early OM-1 cameras had a problem with their tripod baseplate mounts. There were reports of them buckling or being punctured by overly long tripod mount screws that were overtightened. This problem was subsequently fixed in later OM-1 cameras, which have reinforced baseplate tripod mounts. It seems that alot of the issues raised about OM-1 durability arose in very early models, which is not surprising given that the OM-1 design was pretty radical in many respects and therefore had some "teething issues." However, Olympus made numerous improvements/refinements in the OM-1 over time which addressed all of those problems. If I recall correctly, the OM-1n -- the final version of the OM-1 -- had over 30 internal improvements over the original OM-1.
As to OM-1 versus FM, I've used both and prefer the OM-1, which has a brighter, better viewfinder (OM -- 97% coverage versus Nikon FM 93% coverage) and feels more refined. Also, the OM-1 has a 5 frame per second motor drive (both Motor Drive 1 and Motor Drive 2) that outshines the FM's MD-12 motor drive of 3.5 frames per second both in speed, vibration and noise (OM winders though are very noisy and I had reliability problems with Nikon MD-11 motor drive). This difference in motor drive spec does say something about the durability of their drive mechanisms, as faster motor drives put much more stress on the film advancement mechanism.
Both systems have some excellent lenses. I have a good number of the faster OM Zuikos -- 21 f2.0, 28 f2.0, 35 f2.0, 50 f2.0 macro, 90 f2.0 macro and 100 f2.0 and 35-80 f2.8 zoom and can say that you would be hard pressed to find better quality lenses. Of course, Nikon made some outstanding glass too.
However, both cameras are fine tools and which is better is really a personal decision. If you have Nikon lenses or use a Nikon dslr, you should definitely go with the Nikon.
