Of the two main comic shops in Manchester, Travelling Man simply lay out everything on a large table with a snaking going queue around it - you can help yourself to (one copy only) of any title you fancy. They also give out sweeties and encourage kids to dress up. Forbidden Planet is more of a hassle - they tend to get a long queue outside and admit people half a dozen at a time, who get to choose a set number (6 or 8 IIRC) of comics from the titles on offer.
As has been pointed out above, the big corporations simply slap together a few previews and lots of adverts, whilst Tharg produces a quality publication (despite the best efforts of the FCBD people to ban anthologies or large-format comics)- three new strips and a few classic reprints.
People often complain that these freebies aren't officially available elsewhere, but they forget that this event is not about promoting the publications, it's about the retailers getting feet through the door of your local comic shop. Sadly and inevitably, the system is abused by people (and unscrupulous retailers) grabbing large numbers of the best stuff and then sticking them on ebay.
It's a good event for finding things that you would never normally buy - Hip Hop Family Tree was an unexpected highlight for me in recent years.