geoffnewzealand wrote:is centos as stable as they say?
Usually yes, at least CentOS 5 and CentOS 6 were very stable and never disappointed me. Regarding CentOS 7: There seems to be a nasty bug in the current Intel video driver at the moment, and Gnome 3.8 - especially Nautilus - are not as stable as I would like them to have. I guess that's one of the reasons why RHEL (and CentOS) 7.2 will switch to Gnome 3.14 instead, and RedHat is currently fixing bugs in Gnome 3.14 to prepare this migration step.
What I like about CentOS is how they handle point releases. The main system stays the same but certain components like LibreOffice will be updated to a newer, but approved version. This way you will not get the hottest (and quite untested) stuff but will not stuck completely to old versions like you do when using other "stable" distributions like Debian or Ubuntu LTS.
Can I set up a dual boot to try it out - easily?
You can set up a dual boot without problems. Is it easy? This depends on how familiar you are with partitioning your hard drive and using the custom disk layout stuff in Anaconda, the installer of Fedora and CentOS.
Is it a good option for a home desktop for an low-level geek/user?
This is a question I can't answer easily. The problem with CentOS is that most people use it as Server OS. There is desktop stuff in the base repository of CentOS, but it's limited. You will get a basic system with Desktop, Office, Firefox and so on but when wanting "more" (like Pidgin as chat client) you are dependent on foreign repositories. The epel repository tries to fill the gap between the CentOS base repository and the Fedora base repository, but it will not fill the gap completely. Usually the content of epel growes with time and since CentOS 7 is quite new it's not as filled with software as the epel repository for CentOS 6.
Regarding rpmfusion - which you already know by using Fedora - there is a problem: There used to be versions for CentOS but they stopped supporting it. So there is no rpmfusion for CentOS 7. There is a different repository to fill this gap - called nux-desktop - but again, it does not fill it completely.
I write stuff, watch movies and browse - can it do all of that no worries?
Yes, but you have to use the epel and nux-desktop repositories additionally.
I love Gnome3 - is it there by default or is it something new?
When installing CentOS 7 you have the choice of using Gnome 3.8 (will be updated to 3.14 with version 7.2) or KDE 4.10.
So coming back to the initial question: Is CentOS 7 the one for you? I don't know but IMHO it's worth trying out, especially really solid Linux desktop distributions are hard to find. (And IMHO Ubuntu gets worse with release to release.) If there are problems we are here for helping you out.