Which pills are you referring to, specifically? If you are talking about ECA stacks, then I can give you my 2¢ worth.
I have tried Hydroxycut and Thermadrene. I found the Hydroxycut to be effective for me in helping me control my appetite and keeping me from cheating. There's nothing magical about the last effect, just that the pills are expensive somewhat and if I'm spending that much money then I will make sure I exercise and not cheat.
I didn't find the thermadrene as effective, but I was also on a training trip for work, so that may have had something to do with it.
As for fat-blocking/carb-blocking pills, I haven't tried those, but I understand the carb-blockers block mostly starch and not sugar, and aren't all that effective. The fat-blockers would give a low-carber the runs, I would imagine .
I still currently take Hydroxycut, but I'm in a little bit of a stall, scale wise. But that may be due to the Anaprox/Naproxen I take for cramps during TOM. They make me retain water like a sponge, but so far they are the only ones I've found that work, other than Motrin 600mg, which my doctor won't prescribe since not only do they knock out the pain, but thewy knock me out as well!

HTH!
Michelle