Last weekend I was holding a Tactical Shotgun clinic at my gun club. At the end of the day, we were patterning our guns with buckshot at various ranges, and one of the shooters said that he had 3 rounds of flechette ammo he wanted to try. I wasn't expecting to be impressed, but I have to say that this stuff didn't even meet my considerably low expectations.
He fired the first round at a plastic gallon jug filled with water at about 10 feet. This resulted in two punctures in the jug (one side only) with a third flechette sticking into the outside, but not fully penetrating the front of the jug.
We thought maybe he needed to step back a little bit so that the flechettes would have a chance to stabilize. The second shot was fired at about 10 yards on an IDPA target. A few of the flechettes made impressions (sideways) and a couple were actually stuck in the cardboard. No complete penetrations of the cardboard.
Stepping back to 20 yards gave the same result, but less hits.
The bottom line: don't waste your money on this silly ammo ($10 for 3 rounds). It isn't even worth the "novelty value".