GUI - quite good looking but most important, clear and self explanatory.
Sound - now here is where it really got me. As a concept it pretty simple, a filter, a stereo field control, some noise and a (what seems to be) compressor/ clipper, but when you put all those together the result is impressive.
I love how this sounds on drums, it gives instant punch and character. The usual computer drum, a bit sterile and with too much separation between elements, suddenly glue together and give the impression of a single instrument.
On guitars, the filter cut around the "1970" setting most of the unwanted frequencies.
CPU usage - on my system seems to be quite easy on CPU and memory, and I use a mid priced laptop.
Grab it and use it, it's one of the few "vintage warmifier" plugins that adds a useful quality to the sound. Very musical, very intuitive.
Cons - well, not that it affects me, but no Mac version.