I have used TotalFinder for a few weeks now and it has really boosted my workflow. I switched off Apple's "Spaces" for good after only a few hours with TotalFinder.

It runs absolutely stable for me, zero crashes. One small niggle: the "folders on top" option works in list view but sadly not in column view.

However, the tabs alone are such a huge improvement over Apple's finder that I'll happily pay for TotalFinder once it is out of beta.