I'm doing something very similar to what yesno is describing. All of my work files and folders that must be portable are on my iDisk, and I have it mirrored on both the desktop and the laptop. The three way backup that this offers largely saved my bacon a couple of weeks ago when the HD in my iMac bought the farm.

Syncing between iCal Address Book and Mail (the latter which I've only recently embraced) means that my e-mailing experience is identical across platforms. I moved to using Mail after the HD failure. The possibility that my webmail account - of which I have access to no backup - might also be lost some day just makes me weak in the knees.

What's more my business manager always has access to my calendar - she can enter and make changes that I can pick up on the next automatic sync. Added to this is the fact that she carries a PDA synced to iCal, so it's available to her in a portable format as well.

There are some downsides - I can't seem to find a good way to get my iLife applications to sync up - in particular, iWeb (I know, I know) uses it's own special file in the library on a specific Mac, making it essentially impossible to edit a website on both the desktop and the laptop. Also - I started using .Mac this way after the increase in storage - which is great - but I can absolutely see a point at which that initial 10Gigs is gonna start to feel really small - and it gets expensive quick.

I also get what Merlin is saying about changes made to Quicksilver and other, similar apps - since they don't sync, it'll often be several days of frustration while I try new key commands on one machine only to find that I actually changed them on the other

All in all it's worked pretty well - I haven't had the syncing issues that others here describe - but I'd still like to find something that effectively lets me mirror more of my files and some of those app settings.