for hybrid clubs a long time ago.

few people carry anything lower than a 5 iron any longer.

i'm curious about the single length clubsets...i'm tempted to switch out my standard shafts for a set just to try it:

Each tester had more consistent gapping from iron to iron with the single-length set. In two cases, players hit the standard-length 4- and 5-irons the exact same yardage (which is why many people replace their long irons with hybrids).

Players generated faster swing speeds with the standard-length 4- to 6-irons and single-length short irons (8-PW). This is not surprising, given the longer shafts in these clubs.

For the most part, there was a negligible difference in average long-iron carry distance -- two to three yards either way.

Why? Probably because players struck the center of the face more often with the single-length set. (Two testers hit standard long irons a full 12 yards farther.) Single-length 4- and 5-irons launched shots lower. Clubmakers who mass-market single-length clubs will need to address this point. Carry distance was similar in short irons. Some testers were up to five yards longer with the single-length irons, while others hit standard irons farther.
http://www.golf.com/equipment/single-length-irons-against-standard-irons-tested