Worth downloading? Probably, but it cannot replace Ad-aware. Its good to have more than one adware scanner, because each scanner will find a couple things the other didn't. I use Ad-aware about twice a week, and MS Anti-Spyware about once a month. It usually finds 3 or 4 items Ad-aware overlooked.
Yes, go ahead and use microsoft antispyware.In the few tests that i have found to be unbiased, it has outperformed both ad-aware and spybot s&d by a wide margin.And for your information, spybot s&d is mediocre at best.