Do you plan on increasing the volume of PMMA work you do? Are you in a position to take advantage of a disc changer so you can run parts overnight?
The 52s are fine, and if you already have one they can share spare parts and consumables, but the disc changer version (52dci) isn't great imo, it's a compromised design. The prices on the 52 line seem to be coming down, or will in the near future, because it's last year's model. There are so many 52s out there that support and spare parts should be available for the foreseeable future.
The 53 is a clean-sheet redesign that learned from the mistakes of the 52dci. It's got a beefier spindle and more rigid construction, so it's a much more effective PMMA mill than the 52s are; there are roughing tools you can use on a 53 that will cut your job time for big restorations by, I dunno, 2/3rds at least. The disc changer was included in the design from the ground up so it's well-integrated and seems pretty reliable. OTOH there isn't much part commonality between the 52s and 53s, so you'll need to stock two types of spare spindle, two types of replacement collet, two types of replacement ballscrew, etc. Not a huge deal but it may mean having to sink more money into those replacement parts until they need using.
So: if you want a disc-changer dry mill, or if you want to do more PMMA, the 53 is worth looking at. If you want more of the same for cheap, 52d. You can also go non-roland, of course, but once you're in the ecosystem it's easy to just stay there, and you probably won't get as much entry-level bang for your buck as you might with a 52 series.
I talked to a few people who have already bought it, and they were happy with their purchase... So, I thought, for $13K USD, what could go wrong? It comes with CAM included. I'll report back soon after I’ve had a chance to play with it a bit. 
