The results of what looks like the best serological survey, in LA County, were just published. If you take the point estimate at face value, it would put the IFR at ~0.2%, although this should be adjusted for demographics, the delay to death and the delay to seroconversion. https://twitter.com/ewinsberg/status/1252372517366321152

My guess is that, once those adjustments are made, we're going to look at an IFR of ~0.3%-0.6%, but this is just a wild guess, we need more details to do this seriously. In particular I have no idea what the sensitivity and specificity of the assays used were.

Hopefully, the people who conducted that survey will soon publish a preprint with those details, where they try to make those adjustments and properly take into account uncertainty to arrive at a range of estimates.