I'm using jqGrid now. I like it more than Flexigrid for the following reasons:
- Supports trees, sub-grids, and other drill down features
- Many options for in-row and pop-up form editing.
- Several themes to choose from, with support for jQuery UI themes coming RSN.
- There is actual documentation
- It has been very easy for me to customize to do things that neither grid does "out-of-the-box"
Update: I wrote a short article on its advantages and disadvantages in more detail, but with an ASP.NET MVC focus.