They can be effective, but it's not uncommon for people who take them to want instant gratification. I mean, that's kind of the point in using a pill anyways, right? This leads to people taking more than the recommended amount and getting the jitters/shakes to achieve the fast results. It can also go the other way, where the person taking the pills uses them as a counter-balance to bad food. Either way, I tend to believe the pills inherently have some efficacy, but the target demo are apt to abuse or misuse them.
I tried them in addition to working out a few years ago and it curbed my appetite and gave me more energy, but at the expense of bumping up my heart rate, increasing my anxiety, and making me want to gorge myself with food to make it stop. I went through not quite 2 bottles before I decided to throw them away and just stick with diet and exercise.
I imagine they can be pretty useful for responsible lifters/athletes who are trying to get more cut or help shed the last few pounds they're trying to get rid of.