I have two extended bases for my APQS Millennium. I got a Hartley Extended Base as part of the deal when I purchased my machine. I had some issue with the base wanting to pop up when I was using it. But other then that it worked and I was able to learn the basics of ruler work.
About 6 months after I bought my machines I invested in a Ruler Mate from Love to Quilt. My tech support heart just felt like the fact that it is screws to the machine would make it a little more stable then the Hartley product.
The pro for the Hartley base is that it is bigger. Which is nice when you are using large rulers like Crosshatching rulers or large circles.
The pro for the RulerMate is that it is indeed much more stable. Mine never comes off my machine, except for deep cleaning. I have not put my normal needle plate on my machine in years.