All I really want it for is keeping my dynamic range within the bounds of transparency film, namely Velvia 50. Accordingly, I've decided to go minimalist with just one GND of 2x to see how well that works. Beyond that, I'll carry both Astia 100F (to squeak another stop, easily), and color neg film.
The notion of a plastic resin filter that invariably will scratch, and maybe show strange effects when used in front of a polarizer has sold me on the glass rectangular Tiffen GNDs. Just now got a P-series Tiffen XL 2X GND for cheap enroute from Adorama (only place I've found where the extra-long versions are $100, not $150 or more). Also just scooped up a whole bunch of Cokin P adapter rings for all my lenses at fire-sale prices and a Cokin P Circ polarizer for a good savings from Samys auction store.
Likely a big reason why these are getting discounted by the big guys and harder/near impossible to come by at local camera retailers is digital HDR photography. For the cost of one filter one can buy software that'll merge 3 or more bracketed shots. While I personally find most HDR shots are too kitschy, too obvious and heavy-handed in the effect, this may be good news in the short term, for those of us still shooting film-at least until all these filter stocks are depleted.