That issue has raised the most questions. There's been quite a bit of discussion behind the scenes on this. I'm planning an update, but may not have time till after July the 4th.

It appears there's a East Coast / West Coast divide on this topic, with cartridges quite popular on the West Coast, on pools with low bather and debris loads.

I have also sussed out, I think, why some older filter system perform well, when some of new cartridge systems do not. But that will have to wait.

I personally have a particular hatred for the System3 cartridge system, because:
+ The cartridges can be permanently damaged in a SINGLE episode of allowing the pump to run when the filter is dirty (and the lifeguards / pool staff didn't call me). I've got pictures, I'll post eventually.
+ The cartridges are too heavy for many adults to remove by themselves, resulting in damage when a single person removes them.
+ The replacement cartridges are OEM only, and extremely expensive.
+ The cartridges are completely unsuited for commercial bather loads, which even Pentair has now recognized.
+ It's hard to find anything big enough to clean the cartridges in, when they are fouled by suntan oils and lotions.

That said, some people with lightly used pools and a big System3 have been VERY happy with them, because they run all season with a single cleaning done by a service guy.