I have been using the OpenSource Darwin CalendarServer for nearly a year now. I runs it on my old Mac Cube at home (it works on most OSs), to share several calendars between multiple machines. While there is lots of missing functionality, the important stuff works.
I thought I'd give the Google implementation a go, as it saves me having to run my own.
Basically, if you have more than one calendar (I always have had more then one eg. Home, Work, Admin, Holiday, Project, etc.) then you'll find the Google Calendar integration into iCal a real mess.
Unlike Darwin CalDav, a lot of important stuff in Google's CalDav really does not work.
Firstly, if you have multiple calendars, each will be in it's own section in the iCal sidebar, this reeks of a really big hack IMHO.
You cannot make new calendars from iCal.
Each calendar needs replicated account details (username, password etc.).
There is no support for alarms or ToDos etc. etc. etc.
There appears to be such an enormous impedance mismatch between iCal and gCal that I don't see how they will ever get this stuff working smoothly, much like the terrible mess caused when I tried using the new 'Sync with Google' functionality in Leopard's Address Book application. I think Google's models for this kind of info are too simplistic, they only support the simplest possible scenario and then not well.
Since I stopped using the appalling Dot Mac service for syncing, I have tried almost everything I could find to keep this kind of data synced between multiple machines and nothing is really reliable (except CalendarServer). I am not tempted to try MobileMe, it sounds even less reliable then it's predecessor.
It is a real shame, I would happily use Google for this stuff, if it worked with the built-in desktop applications, but GMail is the only Google App that works well enough IMHO. Even then Mail.app coupled with GMail using IMAP can be pretty ropey!!!
So while I am please Google is trying, my assessment is : Could Try Harder.