I purchased a family pack, and have upgraded four Macs successfully so far. In my view, Time Machine is a great idea, and worth the price of admission alone, especially for those who haven't done backups before.
Personally, I use Mozy for my basic backup needs, but like the possibility of additional layers of backup which Time Machine provides.
So, there are a couple of things I discovered:
1. It is possible to trigger Time Machine manually. Simply hold down the "ctrl" key, then click the Time Machine icon in the dock -- a menu will pop up, which contains the item "Back Up Now". This is documented in the online help.
2. Time Machine will not activate itself (apparently) when running on a Mac Book Pro, if running on battery power. It will wait until mains power is connected, and then schedule itself.
3. Time Machine doesn't handle encrypted files well. Specifically, it won't backup individual files stored in an encrypted file system -- instead, it will backup the entire file system. This is not too surprising, considering that backing up the unencrypted files would be a security risk. I guess Apple will be working on some workaround for this, but I don't see an easy fix, due to the key management issues.
4. Time Machine apparently does not use encryption, or even compression, for files stored on the backup device. This is a deficiency in my view which should be corrected in the future, or by third-party add-ons. Naturally this is a user issue -- because the typical user would be unable to deal with the key management issues. I think Mozy has a reasonable approach in this regard, but it's up to each person to decide how to manage encryption keys.
5. The caveat regarding encrypted file systems also applies to virtual machines, which I believe are treated as a monolithic whole -- it's not easy to imagine how this can be otherwise, especially if the virtual machines (I use Parallels, but it also applies to VMware) are not running. Maybe VM vendors will expose their file systems to the fsevents mechanism which harvests file changes in future, and allow Time Machine to selectively back up only changes in the guest operating systems -- after all, FAT32, NTFS and ext3 formats are well-known.
6. It seems the 5160 build of Parallels has an issue with running VMs which are restored from Time Machine. I was able to cause my OS X to kernel panic when trying to run a WinXP which I restored.