I agree, that can seem like the $64000 question. It works for me though. Like many others here I have limited time to devote to riding let alone training so I want to get the most out of it I possibly can. Before getting decent data things were very hit and miss; with interval training in particular. I found it far too easy to end up riding tempo during recovery periods, which meant that I couldn't hold on to a decent pace for the workout phases. To put it another way I went too hard on the easy bits, which meant I couldn't go hard enough on the hard bits.
Real downside to this (if I'm honest) is that the next holy grail is just around the corner (Powertap wheel)[/size].Mmmmm nice
As Frank Zappa once said...the torture never stops, though he might have been referring to something else at the time
Good luck anyway!