It is sad, and dark, but not exactly depressing. It is atmospheric, descriptions of mental states and sapping heat are evocative and feel real. Lessing really makes you empathise with her characters, none of them are clear cut and none are grotesques. I enjoyed it, definitely worth a read.
I had got halfway through Karl Ove Knausgaard's My Struggle, before I began The Grass is Singing, but then I left it on a plane! Along with my Jemima Puddleduck bookmark, which was worse than losing the book. This time I've started Aleksandar Hemon's The Book of my Lives, which is intelligent and engaging so far. If I manage to hold onto it i'll let you know what I think in a few weeks.