Mods supporting sfinv, just to name a few that I've found. Look them up. Use them. Smart_sfinv alone is worth it. The rest just fill out the "game".
mtg_craftguide (from the latest MTG.)
Adding support in your mod, or for a mod that otherwise does not yet support sfinv, is really as easy as registering a formspec, and using the API functions.
In my GAL project, sfinv is included within the gal namespace, gal.sfinv. I found it very easy to modify all the above mods for inclusion in the GAL project, redirecting them to point to gal.sfinv.
The sf in sfinv stands for Super Fast. This is true. The simple, "old", interface can easily be modified, I do believe, just by providing a different background image, in addition to customizing the basic formspec, as an example.
Not to mention, being written by a core dev, it is currently, and will continue to be maintained, where as InvPlus and UnifiedInv are not necessarily.