I disagree with the assessment that its just a few people complaining. Yes, every product is going to have complaints online because some are bound to be defective. However, due to the volume and consistency of complaints about this one specific issue with this one specific model points to a much higher risk of getting a defective mac if you get this model than if you were to get any other model.
Yes, chances are a great deal if not a majority of the 4850's coming out of the factory are fine, but there is no doubt a significant percentage that are not.
This is further evidenced by people who claim to be on their second or third replacement. One defective unit is certainly possible and can happen to anyone. Getting two or three in a row points to something horribly wrong with the product in general.
While forums are by no means a scientific sample, compared to the amount of complaints about other Apple products, especially the new iMacs, it is safe to say that a vast majority of the complaints if not the single main complaint over the last few weeks has been the 4850's freezing. Again, that's not scientific, but its strong enough evidence to me.
My advice is to get a GT130 model, which is still pretty damn good, or waiting until its been verified that whatever plagues the 4850's has been taken care of.