My Experience:

In my experience the upgrade to vCenter 6.0 was fairly painless. VMWare have put in a lot of effort this time around to make it as easy as possible as version 6 has been in an extended BETA cycle.

I would suggest upgrading vCenter 5.5 to the latest 5.5 build (2442329 as of writing) to iron out any bugs patched during the BETA testing of vCenter 6.0 to make the upgrade as smooth as possible.

You will find more detailed and up to date information for installation of vCenter 6.0 from the VMWare site which I would advise you to read before you attempt this.

My aim for this blog is to simply outline the steps I used for my own environment which worked for me. Your mileage may vary and I won't be held responsibile for any damage you cause to your own or someone else's environment. With that said, lets begin :)

This is for the Windows vCenter Server (Not VCSA, which is the SUSE VM Appliance) so if your vCenter is the Server Appliance, some or all of these steps will not work for you.

NOTE: I would advise you to take a snapshot of the vCenter Server before you attempt this in case it fails in your environment. If the VCDB is on a seperate SQL Server (as it should be in a production environment) I would highly suggest you perform a Full Backup of the database prior to upgrading.

Before you begin:
Check out the VMWare Product Interoperability Matrix and ensure any other VMWare products you use in your environment support vCenter 6.0. If not, you will need to obtain the supported versions of these and upgrade them prior to upgrading to vCenter 6.0! This includes any 3rd party plugins such as for Symantec (or other Backup solutions) NetAPP ONTAP or the various HP/Dell vCenter plugins that are out there.

Step 1:
First thing you need (obviously) is the iso downloaded from the VMWare site

Step 2:
Mount the iso to your existing vCenter 5.5 Windows Server VM. In my case this is a Server 2008R2 vm with the latest Windows Updates applied and a healthy vCenter environment with the latest build of vCenter 5.5 installed.

Step 3:
The autorun should run (If not, double click autorun.exe) and you should see a window pop up like the one below:

Simply click Install, then you may be presented with a windows security box, just click run.

You may have noticed, unlike 5.5 and earlier you now have a simple option to install vCenter Server whereas previously you had seperate options such as Single Sign On, the Web Client, etc. That's because these are now integrated into vCenter Server and aren't optional.
Also, Single Sign on (SSO) is replaced by the "Platform Services Controller". New features on vCenter 6.0 can only be administered via the Web Client, as the vSphere client is only included for managing vm's on hardware version 10 and below. So you can carry on running ESXi 5.5 and vm's on hardware version 10 until you are comfortable with upgrading the hosts and vm's. However, you still need to ensure that any plugins and other applications support vCenter 6.0.

Step 4:
The installer will now run and perform some checks on your system.

You will then be presented with the following box. In my case as all vCenter Server roles are all on one box, the installer will upgrade my vCenter 5.5 to 6.0 with the embedded Platform Services Controller. If you have your vCenter Server roles split up (such as SSO) on seperate servers, then the installer will install vCenter 6.0 with an external Platform Services Controller.

Click Next. Then on the next page read and (if you agree) click the box to accept the terms of the license agreement and click Next.
Note: At this point you may get an error if your vCenter Server has less than 8GB Ram. You will need to increase this for the installer to continue.

Step 5:
On the next screen you will be prompted to enter your SSO administrator password. You would have entered this during your first initial vCenter installation (You do remember it, don't you?). Once you enter it (correctly), the installer will run some pre-upgrade checks.

Now, if you have your VCDB installed in MS SQL Express (i.e your VCDB isn't on a seperate SQL Server, most likely because it's running in a lab, not in a production lab, otherwise why would you?!), then you will see the following warning message informing you the installer will migrate the database to VMWare vPostgres:

Just click OK, otherwise you can't continue :)

Step 6:
On the next screen check the port assignments, in most cases you can just click Next.

Step 7:
On the next screen you will need to confirm the install location for vCenter with the embedded Platform Services Controller (if embeddded, this may be different for the External one) and the export location for the 5.5 Database before it is converted to 6.0. This is your chance to move vCenter off the C drive! This I highly recommend doing.

Step 8:
On the next screen you need to tick the box to confirm that you have backed up the vCenter server and the database in order to continue. BACK UP BEFORE YOU CONTINUE!

Once you have backed up vCenter and the database. Click Upgrade and watch the upgrade process spring into life.

At this point I'd suggest grabbing a coffee as the upgrade may take a while to complete!

Step 9:
Once the upgrade is complete, you will be presented with this screen:

The upgraded is now complete! Simply launch the Web Client to play with vCenter 6 (And activate it). You will notice, like I did, things have been moved around and the client also feels a bit faster and more responsive!

Step 10:
You can uninstall the VIM_SQLEXP and SQL Server Express from the server once you see the installation was a success, also don't forget to delete the data exported as per Point 2 in the screenshot above.

Now you can enter your vCenter Server license key in the Web Client and Install VMWare Update Manager (VUM) from the autorun.exe window.

Happy vCenter-ing.