The Radiance hazer is interesting in that it acknowledges that many heater block machines inevitably clog up. It gets round this by using a novel design of heater block that has a series of chambers in it and can be routinely opened for internal cleaning.
http://www.ultratecfx.com/Libraries/...nual.sflb.ashx
The main difference in using a hazer and smoke machine in a nightclub is that DJ's will inevitably use a fog machine excessively to exert their presence. With a haze machine they can't really do that, so the fluid lasts a lot longer.