Yup, it's an old therapy, but can be really effective. If it works for you, and you have nothing to eat except the liquid formula, you could be in remission within a few weeks, and sometimes less. Then if you want, you can continue to use 500 or 750 calories per day of the Peptamen, alongside your regular diet, to help maintain remission longer.
There are a bunch of different formulas available, and they all seem to work about equally well. I've heard some great success stories from parents whose kids have used Peptamen (I'm writing a book about the use of these liquid diets in Crohn's disease, because I think patients deserve to know about this treatment option. As you point out, it can save you from having to use steroids, which is a big advantage.) Good luck if you decide to try it!