Finished the left glove yesterday and cast on for the right.
For a while there I was worried that the thumb gusset might turn out too tight after all, but in the event it's exactly right. And I'm rather pleased with the way the ribbing from the pattern flows into the straight rib of the fingers/thumb. (Colour me relieved, on both counts.)
Preliminary impression of progress on right glove: Seems to be going well. I've done one pattern repeat so far and mirroring the fishscale pattern proved to be easier than I'd feared.