Needless to say, the Kremlin is Moscow's number-one attraction. I visited it in July and must stress some really important things to keep in mind to avoid disappointment.
Firstly, it's the seat of Russia's president, so security is very tough there. Visitors are obliged to leave everything of a size larger than a handbag in a left-luggage room before entering the fortress (prams are allowed). Every damn person is screened at the entrance. No pictures, no videos in any form, and quite a lot of restricted areas. Secondly, if you go on your own, please explore the Kremlin's site (it has a page in English) well in advance as to the way tickets should be bought and to what attractions, check working hours, and be prepared for hordes of people irrespective of the season. Thirdly, remember that there'll be practically no chance for you to grab a bite or even buy a bottle of water once you're inside the complex (toilets are available).
If these recommendations are adhered to, then your visit there will be fantastic. Otherwise, it might be a drag.