When to use LVM?

Having first encountered LVM in HP-UX and now experimenting with it in Redhat and Gentoo, I am wondering, when is it actually good practice to use LVM? Obviously LVM doesn't work for boot partitions, so that question is a pretty easy answer: Not for /boot in Linux.

While trying to figure out how to get my Gentoo system to boot with / as an LVM volume, I found one person who said that they don't understand why people want to put / in as an LVM. His philosophy is that you only put filesystems that you expect to grow in LVM. It makes some sense I suppose. But what about instances where you want to take space away from one filesystem where it's wasted and add it to another filesystem that needs extra? To me, especially for a workstation, it makes a lot of sense to have / in LVM as well as /usr /opt /var and possibly /tmp.

Going even further it appears that HP-UX puts EVERYTHING in LVM including /stand (which is the equivalent of /boot in Linux). I noticed that with their default installation they use /dev/vg00 and break it up into slices for each of the needed filesystems. They also leave unused space to allow the system admin to allocate the free space to any of the filesystems that need to grow in the future. So it seems that this approach is also pretty solid.

I tend to like the HP-UX approach myself even though it can't be done on Linux. Now that leaves me with the last question. If one was going to leave free space for future growth, what percentage would be fair as a rule of thumb? Again, I'm thinking more workstations than servers. I am currently working with a system that has two 80 Gig drives. I've currently it for /boot to be on /dev/sda1 and / to be in an LVM on /dev/sda2. I'm using all the space for / right now... So no future growth there unless I contactenate drives which I really don't trust for reliability.

I think you may be confusing LVM stipulations with filesystem type requirements.
For instance you will notice /stand is HFS While I would bet all your other FS is VXFS.