Heliox is a mixture of helium and oxygen used for very deep diving, usually to greater than 200 feet. Helium's great advantage is that it does not lead to nitrogen narcosis. Helium diving requires as much or more decompression time as nitrogen, so there is no saving there. Beyond 300 feet heliox may cause the 'high pressure nervous syndrome', a shaking sensation that can be incapacitating. Another disadvantage of helium is that it conducts heat about six times faster than nitrogen, so divers get colder than with air diving.
Professional divers can use voice unscramblers to make their speech intelligible.
Overall, helium offers no advantage for recreational divers. Diving with heliox is strictly for technical and professional divers. Trimix is a mixture of oxygen, helium and nitrogen. Nitrogen slows down nerve conduction. Trimix is used for the deepest scuba dives, usually greater than 400 feet.