If you can combine it with fairly intense exercise you can see fairly rapid results but the comments on how people 'feel' are pretty interesting.
Depending on the carb/ratios you play with you can have some pretty dramatic results. Once the body starts to use fats for fuel the highs and lows you used to get tend to disappear. That being said if you have the old carb chaos day don't be surprised if you enter the most epic sugar coma.
Definitely worth a go though, overall it's a really healthy lifestyle choice.