We started with SN100C from the very beginning when we purchased our selective solder machine. It has been working well for us. I can't speak to what happens when changing over to it, but I've heard of many companies using SN100C for selective solder successfully.
We are using SN100C in both our wave and selective machines and have very good results. We also using it for ROHS hand soldering. It flows better than SAC305, the joints are shinny just like Sn63/37. We are looking into using SN100C as our paste but we can not find a vendor who will put SN100C into the cartridges for our MY-500 Jet Paste Printer. We have never had a customer complant with the SN100C joints, in fact they loke the way they look.
SN100C is being used by 95% of our customers over the SAC305. The main reasons are Cost big savings easily $10.00 lb, nice shiny solder joints good for quality inspection. Very few people are using SN100C in SMT Due to increased temperature requirements. Hope this helps