Beaten to it ... but TimC has it absolutely spot-on.
I've been doing these since 1984, probably 25 or so, the last 15 or so consecutively. And over that time the ride has really swelled and the nature of it has changed considerably, it's a lot less pioneering than it seemed to be back in the dark-ages.
All of the preceeding posters are correct, the L2B is eaxctly as described, but in a world where one can decide whether one's glass is half-full or half-empty, one's attitude to the event will shape your day.
It's not a ride for setting a time or a PB as I often hear, it's a day out, a carnival on bikes, a celebration of 2 wheeling, not a day for the budding grimpeur.
For many zillions of cyclist L2B will be the first 'long' bike ride they've ever done and will remain the only long bike ride they'll have ever done and yet for millions more it'll be the first long bike ride of many, a springboard to a life of 2 wheeling.
Rolling down to Brighton will be the super-fit to those that have dusted down their creaky heap in the shed and are littrally struggling every metre of the way to Brighton. All of human life can be seen, there are some supremely unfit making humongous efforts for a good cause and they deserve a little help, support and encouragement.
But the L2B is more than that as well, it is without doubt the biggest, the daddy, the big bike ride that everyone has heard off, it still has kudos with those who'd never consider walking to the shops. It has a buzz like no other ride, it's path is lined by families and suporters, families marrooned in their houses who sit-out and make a party of their imprisonment. It's all the scout eateries that raise money with great butties and cake, it's the kids with hose-pipes filling bottles and cooling-us down, it's the eerie quiet that occurs when people first see Ditchling Beacon in the distance and the utter silence in the last 100 metres before the climb starts, it's the joy on the faces that those that made the climb, the amazing view and the fabulous roll down-hill into Brighton.
Finally, there is nothing else (outside the Champs Elysee on the TDF!) like the throng of cheering people on lining the sea-front, a welcoming comittee for all.
As said, try it once, then decide.