It seems the long sales letter is slowly being replaced by video sales letters.

Personally I think video sales letters would be more effective but they also have the inherent problem that people viewing the page at work (or other similar situation) won't be able to see the video (unless they bookmark it and view it later which is unlikely).

I think combining a mixture of the short sales letter, video sales letter and long sales letter would be a good catch all. Show a clear call to action Buy Now button above the fold with a few simple bullet-point benefits and a sales video. Then below that you can continue on with the long sales letter for those who still have pre-sales questions.