If you haven't tried other brands or a supermarket's own brand then now might be the time to start, you may well spend even less than what's on the special offers. If, at the end of the day you simply don't like the other brands then at least you have tried and saved some money in the process. But try and think about it rationally and work out whether you like the other product just because it's a big brand - there is so in the mind going on here and the supermarkets and brands thrive on this. Supermarkets will have special offers and savings on their own branded products from time to time as well. If these are on what you buy and offer you value for money then it is most definitely worth buying in bulk if you are able to and if t makes sense.
Of course if you have some time to compare prices either on the internet or by visiting different shops on the high street you are likely to save even more but how much time do you really want to spend shopping?