Tim,
I use the Tiffen IR-ND filters. My main use is for blurring moving water in landscapes. For this, you need about 8 or 9 stops total off ISO 100 if lit by direct Sun, less if in more subdued light. The best resolution is always achieved if you limit to one filter, however two stacked on a non-shifting application remains very usable. Hence, my collection is a 0.9 (3-stop), a 1.8 (6-stop) and a 2.7 (9-stop). The issue with the 9-stop is you really cannot see through it well enough to compose when it's mounted. The 3-stop is really only useful in low light or when stacked on top of the 6-stop for additional strength. You can see well enough through a 6-stop to compose, and since most of my moving water shots are not lit by direct Sun, I find it my most-used filter.
You need IR cut ND's because the regular old style ND will leak IR and will color pollute digital images, leaving them kind of a nasty yellow-ish brown color you cannot balance out. Even the IRND versions set color off a bit, but it's easily correctable with a WB adjustment. Best bet is whenever you use the IRND is to shoot a gray card or passport frame for white reference.
PS: Ironically, coatings on filters have a negative effect on resolution and multi-coating is worse than single, but no coating can generate unwanted reflections or flare spots. As such a single-coated option is a best compromise, so avoid all the higher-priced, fancy name multi-coated options. (You can reference a Schneider article on this topic several years back; after which, neither Schneider nor Schott offered MC filters. Tiffen dropped them a few years from later except for video applications, where absolute resolution is not as critical as controlling unwanted reflections and flare.)
Hope this helps,