We also have great success with RBL filtering. Our numbers (while overall smaller in general due to size of our footprint) are in matching proportion to kevind's. The scoring implementation helps us as well, and with a little tuning puts us on the high-side of the spam-caught percentage by a couple points.
We will be implementing rspamd to manage the rest of the filtering - but we would be dealing with a real flood of spam without the RBLs. MailEnable doesn't have the "scoring implementation" that SmarterMail uses, and since we've moved over, SM's implementation has proven to be much more robust.
Many people dislike RBL concept because it doesn't take much to land on one of those lists - but after almost 20 years in this game and only landing on a couple of them once before we locked things down, we've never had a problem since.
MailEnable survivor / convert --