I'm sure I've seen this posted on the forums before but:
http://www.carbibles.com/engineoil_bible.html
This link provides a good over view on the basics of oil and it's job in your engine.
http://www.carbibles.com/additives.html
This is the section on oil additives.
My opinion:
I have never used oil additives on any of my cars. I do however use good quality oil and perform oil and filter changes on my vehicles regularly (about 3,000-4,000 miles). I use full synthetic in my Q. All my vehicles have well over 100k miles on the clock and still ticking away. My 1999 Ford Expedition is closing in on 200k miles.
My point is, it doesn't matter whether an oil additive works or not (the law suits would suggest they don't), you don't need it. Most oil additive companies are marketing products to fix problems that don't really exist. Oil companies spend millions on R&D to perfect their oils. They also work with auto manufacturers to insure their oil products are up to their standards. A good oil (even a conventional oil) will do an excellent job in your engine as long as you change it at the recommended interval.
If an engine is new to you (as in 'used') and maintenance history is questionable an engine flush is not a bad idea. Once sludge starts to form it's VERY hard to get rid of short of completely disassembling and cleaning everything.
So the moral of the story is, if you want to spend money on oil products buy a quality full synthetic oil and filter and keep it changed at the recommended interval. You will get the maximum life out of your engine as well as gain a small amount of additional HP/TQ.