A Q about how good is the Sigma 150mm f/2.8 EX DG APO HSM Macro Lens compared to the Canon EF180mm f/3.5L Macro?

Having both 5DMK2 and 7D I would need to my EF 100mm f/2.8L IS USM Macro a little more tele-macro possibilities - BUT - I am not completely convinced that I could trust the Sigma 150mm f/2.8 EX DG APO over Canons own tele-macro EF 180mm even if it is not equally fast as the Sigma!

Is there perhaps anyone here who have real life field experience of maybe even comparing these two or other tele-macro lenses to this Sigma 150mm?

I just saw someone who wish to sell a Sigma with Sigmas macro-lite flash for this lens for quite reasonable price - so I need to act quickly before this chance is gone!

Cheers,

Charl