As
c-w-s says, the answer to the question depends on who you ask.
Personally I am firmly in the camp of
stewaran:
Originally Posted by
stewaran
For me there are plenty of better things to do with £700
There are endless debates on the value of an Avios, but what some people miss is that everyone values a £ differently too. For example, some posters here may earn £700 in a matter of hours and others may take a week or more to earn that much; some posters may on average be continuously spending £700 every few hours and others may only spend that amount in one go several times a year.
The average person in the UK would look at the J prices and not have to even think twice before choosing Y. If this were not the case then all BA flights would have over 60% of seats in CE. But people on FT are not the average person.
So, if you are a person who sees the price of £70 each way to upgrade and doesn't immediately reject it out of hand, I think you might as well go for it as long as you are clear about the benefits over the Y cabin (which the other replies in this thread have stated excellently).