Pretty good, actually.

There's little doubt NMDA receptor antagonists can cause rapid alleviation of depressive symptoms in a majority of patients who have previously not benefitted much from SSRIs. The effect of a single dose seems to last days, often weeks and sometimes months. Off label use of ketamine is growing because of this.

Now the pharma companies are developing several similar but new (i.e. patentable) substances that they hope have less hallucinatory side-effects, such as lanicemine. Big money is going into this, so the evidence seems to be good enough.