Excuse the basic nature of the question, I've zero experience with regards to this and I'm just looking for a little clarity...

When using serviceguard for a failover from one machine to another, what doesn't get taken from one machine to the other? For example, I was told that OS users and things like /opt don't get carried across. Is this true? Of so, is there anywhere I can get a simple overview of this? I've found the docs on the HP site to be a bit impenetrable, and not explicitly stating what I want to know. I ask because I'm in a situation where some software needs the failover is being set up for needs the OS users as well as certain bits in /opt, /etc and if these don't get taken over as I've been told then that seems a bit pointless.

You should look at this the other way round:
What happens in a failover!
A package moves from one node to another... And so you have to include in the package all it need to run. or have it synchronized on all the servers in the cluster.