Where can I download data from?

You can download data from the respective data producers (RDACs), or as collected by the GHRSST Global Data Archiving Centre (GDAC) at NASA JPL. After 30 days data are transferred to the Long Term Stewardship and Reanalysis Facility (LTSRF) at NOAA NODC.
Real-time (30 days or less) SST from the GHRSST GDAC:

Historical SST (older than 30 days) from the GHRSST LTSRF:

You can drill down to specific data sets by navigating the facets such as resolution or processing level at the GDAC.

How good are the data?

NOAA provides a number of services that can be used to evalute the quality of GHRSST data products.

You can evaluate the quality of many GHRSST Level 2, 3 and 4 producst usign the SST Quality Monitor (SQUAM):

You can also download a collection of in situ data to assess the products yourself using the in situ quality monitor (iQuam):