If it's not tough, like more challenging than anything you've ever done before, you aren't doing it right. I've been incorporating three Tabata rounds into my treadmill routine two days a week and it burns mad calories, according to my heart rate monitor. After doing a good 10-minute warm up with increasing speed, I crank it to 8.5 for the first, 8 for the second, and 7.5 for the third. I rest 1.5 minutes after each one and just jog at about 5.2 to 5.5 between rounds for 45 minutes total. And I'm definitely seeing results.
I've dropped another 1% in body fat since December, but I also started swimming as well so it could be a combination of the two. I've also shaved a good 45 seconds off my mile time. It is a well-researched and proven method, but you really have to go all out and kill it during those 20 seconds to get the most benefit.