I want to sync my iMac with my Macbook, so I'll have a reserve computer if the laptop fails. What's the best way to do it?
Can I have this sync on one profile, and keep working on the iMac as another user?
Syncing your entire hard drive:
One option, if you're not worried about keeping them both completely up to date at all times is to use Firewire Target Disk Mode (if possible) to clone one hard drive over top of the other using a program such as Carbon Copy Cloner or SuperDuper. The biggest caveat with this would be if the computer is newer than the Operating System currently on the other computer. For example the new iMacs have a custom build of 10.6.1 and won't run as they should if given just regular 10.6.1. However once 10.6.2 comes out it's a moot point.
For more actively syncing your home folder I strongly suggest checking out question 40164: Sharing and Syncing Home Folders. Other alternatives not listed in that thread include applications such as Unison.