Likelihood to Recommend
It is not suited for designing site pages and for developing highly customized solutions, as it will crash.
Designer 2010 is a huge improvement over Designer 2007. I'm just now installing Designer 2013 for my next upgrade environment, and I hope it is similarly improved. Given that there aren't any easy alternatives to Designer, it is worth putting up with its idiosyncrasies in order to get quick results for my end users.
But it is a powerful tool, so I will not make it available to the various power users who have requested access to it in production. We can restrict what site(s) Designer users can change, but within those sites, everything becomes up for grabs. Since my company wants to (a) maintain consistent user interfaces, and (b) restrict the number of unghosted pages overall (for performance reasons), we opt to let users from outside my team create their designs/customizations/workflows in the test environment, then we vet them before moving them to production.