Can't speak for the DC version, but the PC version of Sidplay is very close to a real SID on most tunes. On others (eg. the ones by PVCF) it fails miserably though they too sound better than a few versions ago.
The main difficulty in emulating the SID being the analog filters...and resonance. Mostly resonance.
The SID had this unique sound where, if you went too far with resonance, the sound would start bubbling and crackling but in a very cool way. Best example I can think of is the Alien tune from Final Synth II (a compilation of ripped game tunes, I think). Also, I've yet to hear a nice rendition of Theatre Europe... Whew. Maybe it's time the SID programmers create some kind of a fuzzy-logic/random resonance thing.