I'm not sure whether I'm supposed to reply or not since this is Ingrid's forum, but Oscar asked so here goes. With the root/fifth drones, every note you play is an interval, and in fact completes a chord. From an intonation standpoint, this is fantastic, as your ear naturally hears where each pitch should go. It's like somebody gently guiding you down the path. You will automatically adjust your pitch depending on the scale degree, as the pure-tuned position is where it sounds best. In the text I provide a chart showing the CenterPitch display for each interval. This allows you to visually confirm or correct your pitch once you have tuned by ear. Since I incorporated CenterPitch and Tuning Tactics into my practice routine, there is a radical change in the way I hear, and many others will attest to the same.
As Oscar points out, the drones are meditative in nature, and tend to keep you focused and listening. This has benefits beyond intonation, and helps to maximize the value of your practice time.
Mostly I practice scale and arpeggio-based exercises against the drones. It is very effective to play slow lyrical melodies as well. Even dissonant intervals can sound beautiful when well-tuned. I don't usually practice improvisation against the drones (for that I use a metronome or Jamey Aebersold) but there is no reason why you can't. If you want to create drones for specific tunes, I would record the root & fifth of each chord, in time. That would be a very intersting way to expore the harmony.
As far as a 'Welcome' discount, there is a package on the website that includes Tuning Tactics and CenterPitch. When you order, if you mention that you heard about it on this site, you'll receive a bonus copy of my new CD, Perking UP!
Tuning Tactics