I just use Caffeine to keep my Computer from falling to sleep. I haven't really had any problems with sleep mode, other than it coming on too quickly. I'm using the Apple provided keyboard and mouse, so maybe thats why I'm not having any issues. I also have never heard my iMac's fan run at all. I only hear a slight mechanical sounding noise the moment I press the ON button.
It depends what macs you own. The laptops are much noisier than the pc