I've recently (re-)discovered SlickGrid, and am considering using it for a project I'm working on, mainly because in many ways it appears to be the most versatile and powerful and performant (forgive the neologism) JavaScript grid yet created.
There are only a few problems. The first is that it very annoyingly doesn't resize its columns the way users expect: at the same time that they resize the column header. Many would be quick to point out that this has no practical implication, but I value usability and elegance greatly, and this damages both.
As far as usability, the first time I tried to resize a column I really thought it was not working. Undoubtedly others will be confused as well.
As far as elegance, it just completely breaks the sense of actually physically manipulating objects.
I don't actually need all the power that SlickGrid gives for this current project, but if its weaknesses can be overcome I see no reason not to use it for everything when I need a grid. In most respects it is extremely impressive.