Any time that I have been it seemed that the airlines that were going to recruit had just finished interviewing, and the others weren't going to bother. Also, having been promised a reply to letters sent direct to CPs, at their invitation at the conference, I got none. Fifty quid in the scheme of these things is good value, but don't go there expecting to come away with a job: it isn't a car boot sale. What you might learn is who to write to (ie personally, as well as companies), and you might get some CV advice. If it's your first year in aviation having just passed your licence, then I would say definitely go. If you've been around a bit longer, I wouldn't bust a gut for it.