Hi Jax.
Quadrasid is fast becoming my main source for sounds.
There was a MASSIVE improvement in sound quality from v1.5 onward (current version 1.5.1). Refx have incorporated an 'enhance' button, which boosts the frequencies more in line with the likes of 'Vanguard', making it far more gutzy sounding right from the off (no more messing with the EQ to get a decent sound like in previous versions). It now has a very professional, slick sound.
Whilst I agree that nothing can really replace the original SID, Sidstation is far too expensive for what is actually does - it only has 1 sid chip and has nowhere near the control over parameters that Quadrasid does.
Hardsid (Quatro) is better, but a bit clunky to use and VERY NOISY with it's built-in fan. I never really got to using it for composition for that reason - the fan just got on my nerves...
Don't know much about Octasid other than that it's been on the cards for years and doesn't look like it will appear anytime soon (but there will be an upgrade path for Quadrasid users when it does, apparently)... Not sure what it will offer other than more SIDs (obviously) and built in effects.
Even if I wasn't a C64 fan, I think I would still be in love with Quadrasid. I don't know of any other synth (hard or soft) that gives the user so much control over the creation of sounds.
I, (obviously) thoroughly recommend it

Try the demo, if you haven't already...
Tonka