I think I read somewhere recently that a person with 'average luck' investing �30,000 in Premium Bonds should win about 8 prizes a year. Didn't say what amounts.
Currently interest rates are an average of around 3-4% so not as good as some savings accounts - that said you could get the big one - but you could also win nothing.
Have a look at the winners list that National Savings produce every month (on their site) of people who have won over �5000. It seems that unless you hold thousands of bonds you stand very little chance.
Of course you could just be lucky, and furthermore you can always get your cash back.
I've had the max. for eight years or so. In that time I've had loads of �50 wins, the odd �100, and once a �500. Overall, over eight years that is, I reckon I've come out about the same as I would have done in a basic building society account, with the advantage that (a) you haven't got to bother logging the amounts on your annual tax return, and (b) you might always get a bigger win which would tip the balance in your favour.