I have experience with the Si, Sii and Big Sky. The optics on the Big Sky is really good, the image is crisp and clear and it tracks well. I would put it somewhere between a VX2 and VX3. I have not hunted this rifle very hard though and have yet to see how it will hold up.
The Si is on my friend's rifle now and is way better in low light than my Nikon Fieldmaster (called something else in the US but falls between the Monarch and pro staff). The Si is on a 30-06 and have never lost ZERO. My friend hunted the rifle and scope pretty hard over the last two years. The rifle took two huge knocks (he tripped and fell), where in both cases the scope got dented and did not lose zero.
The Sii is also on a 30-06 and has been flawless for three and a half years. The rifle was also hunted pretty hard and fired more than a 1000 rounds.
I think they are hard to beat for what you pay for them.
Pieter
