I would not go there to try on dresses. For the rest of it, if you're after ideas, it's good, and you will be able to meet and speak to a lot of suppliers, though they will all be on the pricey end of the market. It was very busy when I went and so big that I got quite tired by the time I'd been round the whole thing, but it allowed us to get a feel for the market in a lot of different areas. It's worth noting that most of the suppliers are based in the South-East, so if you are getting married far away from London it may be less useful (or you may have to pay for travel expenses).
In terms of freebies, the goody bag was alright (though a few random inclusions like some Tampax and a packet of instant porridge!) but not many of the stalls were giving stuff away. I was particularly disappointed by the lack of cake tasting opportunities - it's all very well having beautiful cakes but I wanted to taste them before considering buying anything! There were a few make-up trials who gave me little demos then tried to sell me very expensive make-up with lots of pseudo-science crap.
I enjoyed it, and if you're early in the planning stages it's probably worthwhile, but the only thing I actually ended up buying from a supplier we met there was our wedding bands. You may want to see if there are any wedding fairs near to your venue as well, so you can meet local suppliers who may be cheaper than the London ones.