I think it's a pretty decent plugin, especially for free. Another excellent free flanger is the Melda one. I use that all the time. 

+1 for the Melda flanger

I've downloaded the Valhalla but haven't tried it yet.

As alternatives, the three d16 modulators are very good and very configurable, maybe a bit too configurable at times, if you know what I mean.

The flanger in particular is excellent. D-16 claim it can do an excellent emulation of the Deluxe Electric Mistress, and it does compare very well with my ancient big-box style DEM and does some nice chorusing and over the top flanger warbling in there as well.