Well, it's quite simple. Do you want a relatively small but regular return on your money or no return but the possibility of winning �1,000,000 every month.
My fiance's parents have a few premium bonds and over the years they have won quite a lot of money with them.The largest amount was �16,000 a couple of years ago.We are buying some for my grandson when he is born in January next year.
I'd say with savings accounts offering up to 6.4% aer on the high street, then you'd need to win more than �1900 per year on premium bonds (assuming the maximum investment of �30,000). If you're looking for tax free savings invest in a cash ISA then try national savings certificates.