After having some trail time with my Pocket Llama now, I can say that the 20" wheels are adequate for cross country riding in most conditions, and are quite decent off-road in many types of terrain.

They excel in tight maneuvering conditions, and track through most types of terrain quite well.

They fall short in off-road use in the following:

deep mud and sand
larger rocks and obstacles which the smaller wheels will try to strike off, rather than climbing.

The 26" wheels are better than the 20" wheels for true mountain biking, unless you're doing a tight and smooth singletrack. Would I race on 20" wheels? No, I wouldn't. No chance. But they're perfectly adequate for expedition / rough trekking bikes, in my opinion.

(I am using Schwalbe Mow Joe 20x2.0 when off-roading)

The 406mm wheel has proven to me to be perfectly adequate for novice - intermediate cross country riding. Avoid the technical stuff.

I think that in my opinion, i'd continue to use the 20" wheel for my purposes quite readily. The largest difficulty is in finding good tires for heavy trekking. For example, from my favourite tire manufacturer in the 26" wheel, there are a bunch of good rough trekking tires - Schwalbe Marathon Extreme, Plus Tour, XR, Cross.

The closest 20" offering is the standard Marathon. Although there are several comments on the Schwalbe website from people wanting a 20" heavy trekking tire, and there seems to be some interest. One can only hope!

Its too bad the snow came back, I was wanting more rides on the little beast