Well, I still haven’t seen it in the US (have you?), but recent benchmarks of the Y500 (I7-3630QM, GT 650M, 1080p) with the docking bay the includes another GT 650M show excellent performance gains over one GT 650M, meaning the SLI actually works well (at least in the benchmarked games).
The GT 650M SLI is actually faster than a GTX 670MX which technically not very surprising as the 670MX should be roughly 50-60% faster than a GT 650M, but still – a very good performance.
Now it remains to see what will be the price for this pleasure.
For 230$ it is a very cost effective addition, but you’ll get the performance gains only in specific games