Well, Id been putting it off for a lil while now, but I finally broke down and ordered a Tiffen. Have a maternity shoot this friday, and the weather is suppossed to be good, so I want to see if I can get some nice washed out backgrounds. My only challenge is that I only have two speedlites, so I guess Ill have some figuring out to do. Wish me luck
Congrats on the purchase! Is it a variable ND or a fixed?
If you're using speedlights then you could also use HSS (high speed sync). I've found if I use too much ND with my speedlights (580exII and 430exII) they have a hard time producing enough power, unless I bring them in VERY close. That's why I tend to stick with strobes (WL x1600 and Einstiens) when using an ND filter.
Using HSS though, the speedlight will do an extremely short burst of flashes, allowing you to use much higher shutter speeds than with a strobe (1/500th - 1/8000th). The only downside is it'll eat into the battery life of the flashes quicker than a normal flash.
Here's a shot I did a while back using speedlights and HSS.
Exif: 5dmkII, 85L, 1/8000th, f/2.0, iso 100
580exII and large silver umbrella high camera right (~7ft from subject) as main light. Bare 430exII next to subject on left (~4ft away) as rim light. The sun is directly behind her providing back/hairlight.
No ND filter.
You might also want to expirement using both HSS and the ND filter and see what kind of results you can achieve.