A.H.
Hi,
I also took the Kaiser classes before my daughter was born in November 2006. I followed my sister's advice and took them earlier in my pregnancy because I was more comfortable sitting for a long period then.
Also, you may want more than a few weeks (or less) to assimilate the information. It may raise questions for you about what you want or don't want in your birth and if you take the classes earlier, you'll have more time to research.
In my opinion, the childbirth prep. was okay, and the breastfeeding class was very good. The most interesting part of the childbirth class for me was seeing the labor & delivery ward where my baby would be born. Seeing it beforehand helped calm my fears of the unknown.
Best of luck!