Well I received my Galaxy SIII on the 30th and I'm very pleased with it.
I was worried that I would find it too big, being used to the Desire, and having quite small hands, but it feels great in the hand (omg I can feel the double entendres coming on again...). As to the 'plasticy feel' I think it feels nice and quite classy in fact.
I've sadly sold on my Desire to a friend. It's nice that I'll still be able to visit it when I want, and keep in touch, and my friend is well pleased with what I still feel is an awesome device.
In reply to the title of this thread, I would definitely recommend the SIII as a worthy replacement for the Desire. It's fast and expandable with no storage headaches, it's got that all-so-essential removable battery, and the developer base is already hitting the floor running.
Over on XDA there is a guide to rooting it using a utility called Odin, Chainfire has ported Clockworkmod recovery, and there is already a custom rom available.
I'm going to wait a while before rooting it, maybe making sure I can return to stock unrooted in case the phone needs replacing under warranty, but I am nevertheless excited about my new phone, and hope that it will serve me as well as did my Desire.
Thank you all for the help you have given freely during my time here. It will be interesting to see where all the Desire's children grow up and move to. A number have already flown the coop and moved to the One X, and now the SIII is out, there is a real candidate for a replacement. I was really considering the Ascend D Quad XL but Huwawei are just taking too long to release it, especially since they demoed the phone months ago, and so now I have made my choice. I may possibly feel a pang of doubt when the Ascend comes out, but I have my new Samsung as consolation. So no regrets here.