Memory Booster is okay - not great.
As to people who say don't use a task killer, they're reciting pap. Task killers are a necessary evil. My Samsung Galaxy hangs constantly - e.g. internet. If I run MB, it starts working again. Unfortunately, Samsung S3 is trash, so I need to run MB 10-20x a day. But that's better than being without internet.
I say MB is only okay, because (despite the settings options), it won't clean cache automatically - thus this trashy s3 still freezes constantly.