While 2 X 20 might not be considered HIIT, it can certainly work into a well rounded program that would be labelled as a "HIIT" program. Most cycling programs are going to include a variety of types of efforts. Those labelled HIIT simply have a great percentage of such work than those considered otherwise.
My apologies if I've confused the subject further. I'm not aware of any formal, published plans that call for HIIT work at the exclussion of all other. The OP even used the term "time crunched" in the intial post, causing me to assume we might consider plans like Carmichael's "Time Crunched Cyclist Plan(s)" as an example of what might be considered HIIT focus training. In that instance he certainly makes allowance for a variety specializations and accommodates those by varying the realative volume of the "intervals" being focused upon.
So, I guess my answer to the OP is, yes, HIIT are effective. Especially so when used in conjunction with other interval types and as much aerobic volume as the cyclist has time for. If I were to have to start cutting any time from my training, it would be from the later and retain the HII's. How you want to differentiate between HIIT, Interval and whatever other plans you're considering is up to you. The grey, crossover areas between those will be huge.
With regard to Carmichael's "Time Crunched Cyclist" I was very disappointed. I had already built an ad hoc plan/program of my own, based on my experience from many years ago and a few magazing articles of his and others from the last two years. While the book answered a few questions and pit falls I was having, I found it fairly basic and it failed to deliver the sort of variety of intervals and how to weight or select from all of them in designing a program that would be best suited to a particular challenge. Instead I found it to offer a good justification for the training approach and spent a lot pages selling the idea of paying for a Carmichael Training System Coach. Oh well. At least it was cheap.
