Let me say that I have been using a hurley 2000 stretcher religiously for the past 2 months. I have gone from 90 to 160 degrees. I found the stretch machine to be infinitely useful because I weightlift, do cardio & martial arts for 1 hour 6 days a week in addition to working FULLTIME. I just don't have the energy to add in a painful stretch workout.
I read Kurtz's book at barne's noble tried it and it didn't work period. Maybe I was doing it wrong, I don't know but I felt like I just wasted time. Luckily I got a full refund...
The side split stretch is one of the most difficult stretches because it is hard to relax and get a full stretch. If you do not have a partner, there is always a certain amount of tension involved no matter how you do it. In my opinion the sitting V-sit is the best way to stretch for the side splits but it is very difficult to hold the legs fully extendedwhen leaning down, your legs naturally want to come together giving you an incomplete stretch. The stretch machine allows you to totally RELAX, and focus on the stretch. That's the key to flexibility, relaxing. If you are tense and fighting the stretch you won't make gains.
If you have a reliable partner or can make stretching a workout of its own, then yes you probably won't need a stretch machine. I don't partner stretch because the martial arts schools around my area are all McDojo's. For me the key to making gains was a relaxed V-sit and the stretch machine has helped me greatly.