Matt Hinrich's story of The Day the Plip Plops Stopped, about his dead standalone TiVo mentions a point that has been making the rounds recently. DirecTV has sold so many new DirecTiVo units for the $99 promotion that they can't meet demand.
I've had regular cable + a TiVo for about two years and a DirecTiVo with satellite for almost two years and there's no comparison. DirecTiVo offers features like two tuners that make program conflicts rare (and you can watch something live while something else is being recorded), the direct storage of satellite feeds means few compression artifacts and a near-DVD quality picture, and put simply, the only way to effectively manage hundreds of channels of content displaying at all hours of the day and night.
I'm hopeful DirecTV sees this demand as a great way to get new customers and keep current ones happy, and keeps the $99 promotion after the new year (when it was supposed to run out).