Suspension systems such as the TRX, Bandit Loops, or gym rings have made a niche within many functional fitness systems. I personally own two sets of gym rings, and found many ways to incorporate them into my sessions.
RING DIPS are one of my least favorite...which means I suck at them. Which also means I do them often. They are considerably more difficult than dips on a fixed-bar. Some trainers estimate the difficulty to be 3-4 times that of their counterpart bar dips. That equates to substituting one ring dip with 3 or 4 bar dips!!!
Gym rings move in what is called a "frictionless plane." The rings require the participant to STABILIZE him/herself during all phases of the movement: top, lowering, bottom, and raising. This stabilization is extension-flexion (front-to-back) and abduction/adduction (in-to-out). It took me almost three weeks of skill work on the rings to even be able to balance myself in the top position without falling onto my face. I would balance for as long as possible in the top position (isometric), then move over to the fixed-bar dip station to go through the range of motion (isotonic).
One of the problems I see with participants on the rings is lack of full range of motion (ROM). There are two points in the movement that can be cheated: at the top by not fulling extending the elbow, and at the bottom by not fully closing the elbow. This shallow movement obviously cheats the musculature that the exercise is made to target. Please be sure to "go deep" and then fully extend to the top. Full ROM ensures the best results.
Whatever fitness level you find yourself in, you can always put dips into your sessions. For extreme beginners, maybe time on a Gravitron counter-balance assist machine is in order. Further scaling options include using rubber assist bands (such as those from Iron Woody). For beginners, I recommend a solid foundation of fixed-bar dips before ring dips. One should be able to complete 10 bar dips before venturing onto the rings. Prematurely getting onto the rings can be demoralizing and discouraging. I do, however, believe in simple balance work on the rings as soon as possible! For those with the highest abilities, try weighted ring dips! I imagine that's about as difficult as it gets!
Last week, I found myself scaling some boulders. I needed the skills and abilities identical to a dip motion to press myself up onto a raised ledge. So is the dip a functional movement? You bet it is.