Now that enough cards have been released to make it possible, I was wondering if anyone has had any experience playing solo-sphere decks. Are they equal to or better than multi-sphere decks?

I think right now multi-sphere decks are still more powerful cause you are picking the most useful cards out of the spheres, where a mono sphere has some situational fluff.

I think Mono Sphere will start to shine after this Adventure Pack cycle is completely released and probably not until we get into the next cycle. At least for solo play.

So far there is little incentive to play mono decks. Currently there's only one card that requires a mono deck (Thicket of Spears). Over time, more cards may be released that make playing mono-sphere decks more attractive.

Anyway, when playing with 3-4 players, mono decks work well enough.