Hi Jason
I've used Scott Shumate's DRO 350 (on an ml7 with glass scales from Allendale, who are great) and a DRO 550 with hacked caliper scales on a mill.
I wouldn't just use the caliper scales 'as-is' because they would constantly be whinging about swarf and coolant. They need to be wrapped up nice and comfy just like the glass scale heads are, so the heads need to be removed from the case and LCD, then supported in a carrier (I used Delrin) which holds it at the right separation from the scale and protects it from swarf etc. Finally the scale needs to be protected against swarf and coolant.
To keep costs down I used the replacement caliper heads from china (virtual village, < a fiver each) and Wixey tablesaw DRO tape for the caliper track, mounted on ally extrusion. The DRO 550 is an excellent bit of kit. Scott developed it as a labour of love and there was so much interest on the Yahoo group that he ended up getting a load of 4-layer SMD boards made and organised a group buy. It's not a commercial product. The deal is you get your DRO board and kit for about £150 including post and customs robbery, Scott makes nothing on it (unless you choose to leave him a donation, which I think nearly everybody did) but if the boat sinks or you fry the board or it's lost in transit, then it's your problem, not his. Some children failed to understand this.
I see on the Shumatech Yahoo group that the microprocessor that he uses is likely to be available again soon, so probably he will be ordering another group buy. If I wanted another DRO I'd be getting my name down on the group buy list.
HTH
Regards,
John