How effective is TRX Suspension Training?
What do you mean by "effective?"
Do you mean, "Does it provide a good workout?" The answer to that is yes-my heart rate goes and stays much higher (remaining above 85% of my maximum beats per minute) than when I am lifting (high intensity, low volume). TRX also allows you do exercises that just aren't possible with your body alone. It is hard work, burns through a lot of calories, and is an excellent workout that allows you to target different muscle groups well.
Do you mean, "Will it bulk you up?" The answer to that is probably not. With all body weight exercises, there are two factors that affect intensity-angles and your body weight. Pulling up from 45 degrees is a lot more intense that from 75 degrees. But the problem here is that in suspension training, there is a limit. You only really have 180 degrees of resistance possible, and your body weight probably is not changing very much, so the only way to progress is by adding volume. However, this added volume is unlikely to result in very large gains in muscle mass at all. So, you'd need to do something like add a weighted vest to increase intensity. The same is true with all body weight exercises, from pull-ups to hanging crunches.
Do you mean, "Will it build lean muscle mass?" The answer to that is yes. While it won't make you Arnold, it will still build some muscle. If you are leaning out, too, then that will be more visible. Building muscle and bulking up are not, in my mind, synonymous. Marathon runners build muscles in their legs, but they don't have 28 inch thighs like some bodybuilders.