At times a one effect DVD can be useful, but at other times it is a simple waste of money.
Some single-effect DVD's are needed because they need to cover a large amount of information in order for you to be able to perform that trick, or sometimes, the trick is just so amazing that 30$ is well worth it. For example, Stigmata is a single effect DVD that's well worth a thirty dollar fee. Because it has performances, advice, the method of the trick, thourough teaching, and also tells you how to perform three different variations on the same effect. another DVD like this might be Ellusionists, Crash Course 2- The Ambitous Card. This dVD covers tons upon tons of sleights, as well as performance theory and audiance management. The trick itself is so amazing that it is worth a fee.
But, other Single-effect DVD's are absolutely terrible and what they basically come down to is a waste of time and money. As I wrote that the very first DVD that popped into my head was Ellusionists: The Axis change. The trick itself looks very good in the preview, but in real life it could never be performed due to very knacky angles. The DVD has limited explanation and the trick itself is impractical for real life use. So this would mean that you would be throwing your money away for a DVD like this.
Dylan P.