How painless is the transition with a fairly large collection of custom filters and the like?

If you use custom filters instead of subscribing to one of the popular filtersets, then you should check to see whether you're even using the kind of rules that ABP doesn't handle well. The memory and CPU usage only gets bad when you've got a lot of broad-spectrum CSS element hiding rules; regular URL-based blocking rules are plenty scalable.

I don't have any custom filters, but AFAIK it uses exactly the same format for its black lists. The "my filters" pages says:

> A filter can be a plain hostname, or an Adblock Plus-compatible filter. Lines prefixed with ‘!’ will be ignored.

For people complaining about the memory usage and using adblock, uBlock really solves everything, it's really night and day.