I'll (M31) throw in my two cents here as someone who has been training this way for about 12 weeks now with some apparent success. First off, I believe this way of training is best suited for the middle distances 1500m - 5000m and is probably most effective for people more on the fast twitch side of the muscle fiber distribution spectrum because it allows them to get in high volumes of quality running without the wear and tear that would come from long, sustained tempo/threshold efforts. For a "slow twitcher", longer sustained efforts may be more beneficial.
When I started this journey, I had recently run a road 5k in 15:30, so that was my baseline fitness off of more traditional training (60 - 70 mpw, 1 long run, 1 track session, 1 threshold run). I took a chance on switching to this type of training and pretty much overhauled everything at once, jumping right into doing 6x1k (3x/wk), 10x400m, 15x200m, one rest day, and one more traditional workout or track session with my club. With all the warms ups, cool downs, and recovery jogs, I have averaged 50 - 55 mpw each week for the last 3 months with very little variation in the weekly total. I've run two races in this time including a 3k in 8:42 and a hilly 10k road race in 32-low, winning both easily. Both of these performances are superior to my 15:30 5k, which was done on a very fast course.
I did make a few mistakes early on which resulted in having to dig myself out of a hole a couple of times. By far the best advice you will get about this method is that you need to keep the warm ups, cool downs, and recovery jogs EXTREMELY easy, and only focus on running fast during the quality portions of each interval session. Also, I had a lot of pride during the first few weeks so I excluded the 15-20 second walking break after each rep. Big mistake. Remember, these sessions are meant to replace your more traditional easy runs and should not be stressful.
I've been using my weekly harder workout or track session with my club as a sort of fitness check each week in lieu of racing. The last 4 - 5 weeks I have done the following sessions:
4 mile tempo on the road @ 5:12 pace + 5x400m in 65 w/ 1 min rest
1 mile at 95% effort in 4:28 + 8x400m @ 66 w/ final rep in 59
8x800m off 2 min rest averaging 2:21 (first rep 2:27 working down to 2:19, 2:16, 2:12 on last 3)
I could never have done any of the above 3 workouts on my old training without racing the session. The most incredible thing about these efforts was how incredibly controlled and fluid I felt at these paces. I am running another 5k road race this Sunday (first race in 6 weeks), so I will report back to this thread as it should serve as a pretty good sign of progress (or lack thereof) compared to the 15:30 fitness I had before starting this.