I don't think anyone has done the research to see how many of the submitted rumors actually turn out to be correct. There are over 1800 submissions and I'd say 60% of those, at least, aren't rumors or repeat what someone has already said. Still, too many left to go through.
It's hard to say in terms of % correct. Because there are so many small iterations and even stuff that might be far ahead of time.
As long as you take things with a grain of salt... and take into account sources... I think rumors do a pretty good job knowing what's in the pipeline.
Probably a small portion. About 80% are ones that are implausible (6ghz G5, iWalk 2.0, Apple Segway, Disney merge) then about 20% are ones that can actually happen and out of that 20% I would guess about 20% of those are true.