Thanks TonyK, I am still thinking about it, like you say, I don't really need to do it, and I am normally on the side of if it aint broke, don't fix but... You know what it is like, you hate to think you are missing something. It seems that it either goes really well or really badly!!!
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After going from Mavericks to Yosemite in an upgrade without problems I was forced to clean install El Capitan since the upgrade resulted in a system not shutting down in the normal way but only by removing power supply.
I entirely agree with those suggesting to clone your existing drive in a bootable external hard drive and upgrade THAT.
Even if it will be slower than an inner drive, it will show you what works and how well it does...or doesn't.
I however
do not agree with the opinions that OSX 10.11 will either be a great or awful experience
For a normal non/pro user like myself I could use Mavericks or Yosemite not remarking significative differences besides that the design is a bit different and the Mavericks icons and dock were IMHO much nicer.
As to TM I would suggest it only additionnally to a bootable clone drive.
After one (in my case unnoticed) backup in El Capitan, it was no more usable with all the previous backups covering a very long time.
So removing it is a must.
Ed