Is inline edit good UX? Which is a better way to edit a grid, providing an inline edit option or providing a view below the grid, (display additional details and provide editable fields)?
There's 2 factors to think about when deciding which type of editing format to use: Speed of Update and Complexity/Dependency within the data.
Speedy entry of simple independent data: use Inline Edit
If speed of updating individual pieces of information is important, then inline editing hands down beats editing sections. User can click into the section and directly edit a cell. Whereas with section edit, the user needs to reorientate themselves to find the field of interest before they can edit it.
Editing of more complex dataset with dependencies: use Section Edit
However, if the data you're dealing with is tightly coupled to other data fields and/or user tend to edit the info together as a whole, then section edit may be a better choice. Showing data as a group makes it easier to present dependencies (e.g. the data in one field affects valid values of another field) to the user and allowing them to better edit a set of fields as a group.
I think: In systems where forms or data grids are used by Account Managers or data entry personnel, Inline Edit is very useful. Opening a Modal or a an expanded view creates multiple clicks for the user and is considered time consuming.
When the same information is presented to the End user, a view or modal may be the right thing to do.
Also it totally depends on the number of editable fields.