I went to The Baby Show back in February, before DD was born, and found it pretty useful. :)
I'd done my research beforehand and knew which cot/travel system/etc I wanted. I'd also found out the cheapest I could get each product for online.
Armed with all the info, I turned up at opening time on the second day. Initially, for the first hour or so, it was pretty quiet. However it got steadily busier and was heaving by lunchtime when we left.
I got to have a good play with the travel system I was intending to get at the maker's stall, then ordered it for a better price than available online at a different stall, with a few nice extras thrown in. Same with the cot - £120 online, but I got it for £80 from the company direct with a mattress thrown in.
I also got a really good deal on nappy bins and have enough cartridges to last until next year. I got some good quality cot sheets and blankets at a good price too.
The Mothercare stall was a waste of time, they had barely any of their stock and just a flat 10% discount, which you can get online with codes. The staff also had no idea about the products they were selling and had to keep referring to the catalogues for information I could have read online.
Philips was pretty bad too, the staff were knowledgeable but their deals were for fixed packages and they couldn't be flexible over it. i.e. You had to get the Electric Pump with the Electric Warmer and Steriliser to get a good deal, you couldn't get the Microwave Steriliser instead. They'd also already sold out of most of their good stuff - and this was only the start of day 2!
So basically, if you only want a few little things or to window shop, it's probably not worth it. But if you have some big purchases and know what it is you're after, and the going price, then you can save yourself quite a bit and/or get some freebies thrown in. :)