Its pretty efficient, as long as you maintain the compressor and keep the proper oil level in it. You never want it over filled with oil, nor do you want it under filled. Right in the middle is good, around 8-10oz. The easiest thing to do on a 4.3 S10 is to buy brackets from Suicidedoors.com (way cheaper than airlift), and get a Mini209 from a junkyard. Lots of early 80's Audi's have them, the 4000s in particular.
Their output is 100% dependent on engine speed and pressure. The faster your engine is going, the faster the compressor will output.
I wouldn't ever run an engine driven without some sort of electric backup however. If you piece together your own kit, you're looking at around $400 for everything, right around the cost of two Viair compressors, or cheaper if you get the Dual packs. However the benefits of always having air are oh so good.
Check out
http://www.airridetalk.com/tutorials/tutorials/air-on-demand/