Yoga? Is it really exercise?

I signed up for a yoga class.  It begins on Monday.  Everyone always raves on about how great yoga is, but...  I've seen it done.  It doesn't look that...  what's the word...  difficult...  But everyone tells me I'll be surprised.  Do you guys seriously consider yoga an "exercise"?  Will this be something that continues to promote my weight loss or is it just kind of a lot of breathing and stretching.  ;)  I know I sound ignorant about this but I really am excited to try it out.  I'm just wondering if it's going to be worth it to ditch a day of weight training for this.  Suggestions?  Comments?  Experiences?

yoga looks incredibly easy, but is SO not. it's great, but it'll kick your butt. movements are all slow and methodical, but they'll have your muscles aching and you'll (believe it or not) work up a serious sweat... you'll come out feeling refreshed and accomplished though... i bet you'll love it...

Yep. You'll get a work out. kp