Anyone have any experience with BigWorld?

They've been around forever, I think "World of Tanks" saved them.
Before that trilogy, they didn't really have any marquee names.

As in BigWorld server...how well does it work? It can supposedly support single shards with large player counts.

I've not used it, but 10 years ago I had the impression that it was pretty rubbish.. however, since then World of Tanks used it and made so much money that they simply bought the developer of Big World and took over. Since then, it has likely improved at a phenomenal rate (out of necessity -- it does manage to operate a legitimate empire of massive multiplayer games)