My brewery is predominantly stainless. I have a polycarbonate filter in the brewery, a HPDE 60ltr fv, and 2 x 50ltr stainless fv's. I intend to get a conical very soon but the jury is out as to whether to go for HDPE or stainless.
I started out with a plastic brewery but found limitations I wasn't prepared to accept with some aspects of the gear. Boiling almost 30ltrs of wort in a plastic bucket with a wobbly tap is not a good idea, as I tend to be a little clumsy in the brewery. On more than one occasion I almost knocked the tap off, and that would have had really serious consequences.
I also prefer stainless from a cleaning point of view. I don't have to worry about scratching things (important in FV's).
I use stainless in my brewery now as I like my gear to be robust, and also because it is almost totally inert regards reacting with cleaning agents I use, and the brewing process. When I set out making the present brewery it was with the intention of it lasting possibly ti'll the end of my days, another reason I went for stainless.
I had an interesting conversation recently with a well regarded home brewer. It went along the lines of him saying my gear was completely OTT for a home brewer. My reply was to liken our gear to cars. You may drive a Ford Ka or a BMW, both do exactly the same job, but you wouldn't walk up to each other and start criticising each other cars, so why would you do it with home brew gear!