Book Fairs

If this has been discussed before, please forgive me - I have just joined the group today.
Does anyone have experience of attending one of these things (book fairs I mean)?
The reason for asking is that the deadline for applying to the forthcoming one in London is 27 March so there is very little time to decide whether or not it is worth doing, particularly from a financial point of view.
I would be hooking on to the Trafford site, which would set me back around £250, but they do (I understand) have a pretty good display and authors would be represented by Trafford's own staff.
What do you think?

Ralph Fairhead