If I am thinking about the right mill here I believe this mill is more rigid than a Z45. There very well built and fit a small shop real nice. Now I do believe they are limited on table travel compared to a Z45. Also it seems those mills had a very nice spindle RPM that goes way higher than the Z45. A lot depends on your needs but I like these little Rockwell knee mills and I came very close to bringing one home before I bought my RF-45 clone mill. Machine to machine I think the Rockwell is a much better built unit and it was hard for me to pass it up. The table travel is why I did though and if it had not been for that it would have been hands down a no brainier between the two mills. Just look at the construction and then add in the 3 times faster spindle and its easy to see just how nice these little knee mills are. For the most part they are built much better than most of the bench top mills and they last very good.
I wish I could find a pic real fast but the one I am thinking about has the Grey with white trim paint. Seems like they are a 21-122 series or something like that. When it comes to CNC the 45 clone mills are not that easy to CNC. There is a few parts out there to do this but its really best to build a kit to do one. The Rockwell may still be harder to do because of the knee but if a person needed a mill with this much strength and could deal with the small table travel it would make a very nice CNC setup. I don't think I can recall seeing one CNCed though, and too CNC one everything would have to be built from scratch but thats no big problem really. It would be best to add some kinda air cylinder to assist the knee like the Bridgeport guys do but even that would not be much a problem as long as you have air in the shop.
Now even though these mills are built a bit better, they still don't have a hugh amount of mass/weight going for them. There not a Bridgeport. But all in all there a nice little package that can preform quit well if in good shape. Price, and my intended needs would go a long way into choosing between these mills. Because the truth is you can get a Industrial Hobbies mill that is plenty strong enough with maybe even twice the table travel at a very good price. You will find its easy to out grow a small table mill, but then again it does depend on your needs.