I've got the salt water test strips that duraleigh carries, and last season the strips were spot-on with what my SWG was reading.
I had to replace the SWG cell in the off-season, so the cell could be partly to blame here, but this year, the same test strip bottle was reading 4700+ when I tested, while the SWG cell was showing Low salt (which is under 3000, if I remember correctly). I bought a new test strip bottle, and the new strip is showing 3700 or thereabouts, so that means it disagrees with the old strips by 1000 ppm.
And even though the new strips are showing 3700, the SWG cell is still showing Low, when it should be showing Good. I'm afraid my SWG (Intellichlor) may not be producing much chlorine as a result, as I'm right at 2.0 ppm CL, on the low end of the recommended level.