Disclaimer: I’ve only seen the British version of the show, and haven’t seen it for a while - looking at the Wikipedia article, I was surprised to see that Trinny and Susannah are no longer presenting it!
But when I did watch it, I thought it was mostly good advice, with some of T&S’s own biases mixed in, of course.
As I remember it, most of the women (and occasional men) they gave makeovers to fit into one of two groups. The larger group was dressing, they thought, to hide what they perceived as their flaws. Unfortunately they were failing to bring out their good features (they often didn’t believe they had good features!), usually failing to conceal what they set out to conceal, and generally made themselves look heavier and considerably less attractive than they really were. One woman I remember clearly, for instance, had recently lost a lot of weight and was proud of it. But you wouldn’t guess that from the way she was dressing, still wearing the tent-like styles that hadn’t flattered her when she was heavy and now completely hid the attractive figure she’d worked so hard to acheive.
Another group was dressing as they had when they were younger - either because they believed that dressing like a 25-year-old (or their perception of how a 25 y.o. should dress) would make them look younger, or because they believed that was “their” style and newer styles were not for them.
This doesn’t cover all the people who were made-over, but I think I can say those were the most common reasons why friends brought them to the show’s/T&S’s attention. For the first group, the advice dealt with ways to emphasize their assets and to a lesser extent de-emphasize “problem areas” in order to draw the eye to what was beautiful about these women. For the second, they emphasized finding classic styles that flattered each woman and would make the best of who she was right now, rather than trying to hang on to her twenties. In most, though not all, cases, I thought the women looked much better when wearing the clothes that fit the “rules”.
Neither one of these things is difficult, really, but in my experience it isn’t what most women talk about when they talk about clothes. Many women figure these things out by themselves, sometimes with the help of a friend or two, but quite a few just keep buying the fashion magazines and listening to friends with different body shapes and coloring talk about what they buy, and then figure something must be wrong with them when they don’t look as good as they’d hoped.
(But I thought the “throw it all out” rule was/is dumb. I’m not going to wear my newest and most flattering clothes when I’m cleaning out the attic, y’know?)