I visited some of the palaces without a guide, others with a guide. If you have a lot of time and have already done a lot of background reading, and also have specific things in mind to see, then it's probably better to go on your own. This will give you the freedom to spend as much or as little time as you want. Belochka is right, a good understanding of Russian would probably really help, since many of the palaces do not have signs in English. But again, if you already know what you are seeing, then it wouldn't matter that much. True about the CP being too crowded in the summer to attempt buying a ticket on your own. AP, I think, is a different story, it doesn't seem to be too crowded even in the high season.
Other palaces, like the Yussupov Moika place, has gotten very popular recently and tickets are needed in advance during the summer too. I think in order to see the "Rasputin cellar" you have to go with a guide.