I bought a few el cheapo Hong Kong NDs recently from Ebay, to have a try at long exposure daylight shots. It was a "kit" of three @ 72mm; ND2, ND4 and ND8, but they don't appear to be as "stoppy" as I'd have expected. Even the ND8 only adds a couple of stops-worth of exposure time. Also, the threads are "varied" (polite version
) in terms of quality of machining. I thought I'd try these first (only £15 inc p&p!) before going for any of the big name versions. At over £100, I'd be hard pushed to justify one of the Lee Big Stoppers. That's just crazy money.