My new one on Projekt is in a 4-panel ecowallet (no plastic), with a matte finish. I think this is part of Projekt's move away from the traditional jewel box. Digipacks are nice, too, but you have to be careful not to ding them.
Having provided art for this lovely disc (and a few others in the format) I must say it's a favourite! Though the ultimate hybrid is the Japanese edition of "Ocean Fire" by Willits/Sakamoto. It's like an ecowallet/digipak with a normal spine width and a flawless and secure way of holding the disc in place that I've never seen elsewhere.
The issue of archival survival is rarely addressed. I fear my recent Darla discs using the recycled board for artwork will not fare well. Mind you, some old jewel cases are made of dodgy plastics and in one case, a CMP jazz disc's white plastic tray went yellow and affected the paper stock!