I went to the one in Birmingham last year, the adverts and facebook promos tell you how great they are and almost sell it as though you are guaranteed a job there and then! In my experience there are about 20 stalls mostly comprising of companies trying to ship your goods or convert your money, and a few stalls with horrendous queues that take your details, tell you everything you know already (very basic info) and then you never hear from them again.
I know this is another doom and gloom reply, and I'm always willing to give the benefit of the doubt, but it really is a waste of time and money. To be completely frank, I think it is an insult that they actually charge for what is basically a sales event!
I went expecting to fill most of the day, I saw everything in 15 mins and was only there for over an hour becasue I had to queue for so long!
The best event I went to was at the Australian embassy (in that London), I got an invite from the Institute of Civil Engineers (I'm a Civil Engineer), it was free, there was a seminar by the Immigration Dept, followed by 30 or so stalls including a representative from every state, lots of agencies and employers.
That was the turning point to make me decide I wanted to emigrate! Keep an eye out for those events, you'll probably get more info from the imm.gov.au website.