I've had a chance to try out the new Laowa 25mm 2.5-5x macro for a while and have written up some details about using it, including a brief comparison with my Canon MP-E65 1-5x macro.
http://www.northlight-images.co.uk/laowa-25mm-f2-8-macro-review/It's a solidly built bit of kit - fully manual though, so the lack of auto stop-down may be a hindrance to some.
Wide open, the quality is very good on my 5Ds, pipping the MP-E IMHO, particularly when stacking a lot of shots at small intervals
