If you do opt to stay at the NEC itself through the CC then just check beforehand about the price and facilities.
We found that it was more expensive than some of the sites, we were told to bring as much water as we could and there were only limited facilities for disposing of toilets, rubbish etc.
It may have changed since we enquired but because of the price etc we found the caravan sites a much better option.
I can see the advanges if only staying overnight but more days and I would prefer a beter place than a piece of concrete outside the NEC