Better Family Photos: Fotodiox Generic Cokin P type Filters

Summer's around the corner and in anticipation of more outdoor shots, I bought filters. I'm not into landscape photography, and I don't expect to use them very often, so I couldn't justify spending a lot on them. I turned to ebay and found these filters from Fotodiox. The "Cokin P" style filter holder is made by Tian Ya. The filters have no identifying marks. I called Fotodiox to ask them if they are the same as the ones by Tian Ya. Fotodiox isn't familiar with Tian Ya, but did confirm the filters are made in China with CR-39 plastic.

Link to album with test shots
Update: real world shots with circular polarizer
Update: sample shots with graduated ND filter

Build quality
Build quality seems reasonable for the price.
The adapter (which is screwed onto the lens) is made of aluminum. The threading seems ok - I attached it to my lens (Tamron 17-50 VC) with no problems.
The filter holder is made of plastic (ABS plastic?). It looks like it will withstand normal use. It fits onto the adapter fairly well (it doesn't rattle or anything).
The filters are plastic (Fotodiox states that they are made of CR-39 plastic, the same as some of Cokin's filters). Other than the circular polarizer, they are all cut to the same rectangular dimensions: 4 inches x 3.5 inches (actually 3 15/16 x 3 6/16).
I've read a criticism posted on photo.net forums about the filters being too loose in the holder. I didn't find that to be an issue with my holder and my filter. I didn't dare shake my camera and my lens to test but I shook the filter holder with filters inserted. It seems to me that normal shaking from ordinary use will not cause noticeable movement in the filter. A vigorous movement, as though you're throwing a paper airplane, can cause the filter to move (about three such jerks would be sufficient to dislodge a filter). Jerking the filter holder extremely hard like your life depended on it can dislodge the filter in one jerk.
Update: because the filters are rectangular and are larger than the filter holder itself, the filters can get pushed accidentally in crowded areas.

Vignetting
Although the P series is advertised for a 35mm focal length equivalent of 28mm or longer, I can use it on the Tamron 17-50 with no apparent vignetting even at 17mm (25.5mm equivalent) except when the holder is tilted at a diagonal. When the filter holder is at a diagonal, vignetting is visible until about 25mm (37.5 mm equivalent) when it becomes very slight.
Baseline shot at 17mm (no filter holder):

Test shot at 17mm (filter holder attached, level):

Test shot at 17mm (filter holder attached, tilted):

Test shot at 25mm (filter holder attached, tilted):

Accuracy of Exposure Reduction from Neutral Density filters.
I also wanted to know if the neutral density filters reduced exposure to the indicated extent. I took test shots of our white garage door in manual mode, JPEG, picture control: neutral (default settings), white balance: custom. Briefly, the ND2 and ND4 are reasonably accurate. The ND8 acts almost like an ND32 (4.6 stops reduction).

Baseline shot (no filter) at ISO 200:

Test shot with ND2 (1 stop reduction) at ISO 400:

Test shot with ND4 (2 stop reduction) at ISO 800:

Test shot with ND8 (3 stop reduction) at ISO 1600:

Test shot with ND8 (3 stop reduction) at ISO 5000 (2/3 stop more than 3200):

Test shot with ND4 and ND2 stacked, at ISO 1600:

Comparison of ND8 (top) with stacked ND4 and ND2 (bottom):

Circular polarizer's effectiveness at filtering polarized light.
To test the circular polarizer's effectiveness, I took test shots through polarized sunglasses and tested how opaque the sunglasses became. I found that at the peak of the polarized light filtration, the CPL blocked about 7 stops of polarized light.
CPL test shot through polarized sunglasses - minimum filtration, ISO 200, f/4.5, 1/250: