It is not a particularly selective PKC alpha inhibitor, but in cell free assays the IC50 values are: 2.3 nM for this compound at PKC alpha and 6.2 nM for PKCB1, from http://www.jbc.org/content/268/13/9194.long
I used it for pan-PKC inhibition at 1-10 uM for in vitro cell based assay.
Agree with John. It's not terribly specific, but most kinase inhibitors aren't. Philip Cohen at the University of Dundee has an incredibly useful series of papers examining the specificity of various protein kinase inhibitors.
Even at 1 uM, Go6976 still hits MAPKAP-K1, MSK1, PDK1, S6K1, PHK, and CHK1. I would use it in the range that John described, but be sure to couple the studies with a second method of verification like siRNA.