I am not an expert in the C64 SID chip, but I love retro gaming sounds. I can't say if Retro-GS01 is close to the SID, but it is definately a unique sounding instrument.
The sounds are great. If you are a fan of QuadraSID and find it a bit difficult to program, try Retro-GS01. It is set up a bit more traditionally. You can make everything from fat harsh sounds to softer whistle-like sounds. And where some VSTs can sound thin and transparent, Retro-GS01 never seems to. I bet with the right effects or several instances of it running, you can get some pretty big sounds out of it. The presets provided with the demo show the wide range of sounds you can produce. However, the presets that are included with the full version are even more impressive.
The GUI is nice and inovative. Not only do the knobs have numbers in them to show the level they are at, they change color to show when they are in their original state. This is a nice touch that would be handy in other synths, visual coolness that serves a purpose. The colors are nice and bright, almost like candy. The only problem is the green is a bit bright compared to the reds and blues (could be my monitor).
Retro-GS01 is a wonderful sounding instrument that is easy to use. It is very stable and uses little CPU. Although it is based on the classic SID sound, it is capable of an even wider variety of sounds.