I was wondering who outbid me on that one yesterday. LoL.
The Tokina SD version got pretty good reviews on several searches I did. The complaints were about slow AF and not being super sharp without good light. However; if you read close, most were comparing to much higher costing lenses. A Nikon 200-400 f4 or 500 f4 it's not, but given its cost/performance ratio, I think it's worth it. It's also supposed to be much sharper and faster than it's Sigma APO counterpart.