TreeGrid is pretty easy to extend by overloading their API events.
It is also pretty easy to configure a grid to suit your needs using xml configuration files, overloading defaults, actions, events...although it takes some time getting used to the special format of this xml, since they use xml attributes to specify the properties of the columns on a grid, and the way some things are done (calculations, editability...)
It offers endless configuration options and a ton of functionality, it is updated regularly and the support (if you pay for such license) is quite good.
As for how easy it is to maintain the code, if you program using the API they provide (I think there are different licenses that expose different amounts of the API, so be careful there), it is straightforward, since most of the details are hidden to you. If you are planning on buying a license including the source code (which is possible) the maintainability of the changes is up to you and your coding practices :)
(Note: I m not affiliated to CoqSoft or anything, I just happen to work with their component)