I've done a few and I certainly wouldn't want to do one on an MTB with slicks if I wanted to finish on the same day as everyone else.
A full carbon road bike, deep section rims, replica team kit and carbon disco slippers is the only way to go and don't expect anyone to talk to you if your groupset is any lower than Chorus or Ultegra!
But seriously though, if you fancy giving it a go then go for it. Yes they can be a few quid but the ones I've done have been well organised with feed stations and a well marked route of over 100 miles on mainly quiet roads that loop back to where you started. Don't listen to the guys who said you're paying £20 for a couple of bananas as they were clearly exaggerating, they have flapjacks & Lucozade Sport as well!
If you want to really test yourself though, hop across the channel and do La Marmotte.