Personally, I don't use them to much. But I've tried them out and can clearly see their benifits.

Von Milash - agreed, isometrics can be harmful if you don't "remind" the memory.

Adam - you can use them for upperbody by holding the isometric position in several angles. For example a jab isometric position:
#1 - from the position of the stance start the resistance (focus on going to 100% effort in as little time as possible), hold for 5 seconds
#2 - hold the position of half-extended jab for 5 seconds (again focus on 0-100% in as little time as possible
#3 - same except your hand is almost completely extended
Then throw several jabs in the air to loosen up and remind muscles of the movement



I also think that gymnastic holds (such as on rings) are great strength builders. They require loads of core strength not to mention upper body strength. And they only last up to 10-15 seconds. 3 sets normally. That is very little time and since we are always pressed for it I think gymnastic holds would be a very good addition to our workouts due to brevity, usefulness, and intensity on body as a unit (notably core).