I suppose you could do a search for particular magazines to see if any major wins were posted, and you probably wouldn't want to make a decision based on just the three replies above...but, yes, for the time you spend entering it is probably not worth the effort as so many people enter magazine sweeps. There is no real formula to give us an answer, just opinions and attitudes. Usually, I like the (flawed) optimism in "If you don't enter, you can't win," but that is not the whole story.
There are so many sweeps to choose from. Enjoy and good luck!