Users, in general, hate applications that are so poorly thought out that they have to customize to get their needs met. In my experieinceexperience, most users are not as comfortable using a computer as an IT person and will not customize even when given the option.
If you are customizing in order to avoid doing a proper job of designing, you are customizing too much. If you are customizing because you like customization and not becasuebecause it is a requirement, you are customizing too much. If you have a genuine requirement to customize, you should still question if it is a good idea and find out the underlying reason why they think they need customization. Often you will findafind a new requirement they hadn't mentioned if you use a requirement for customization as a place to start a dialogn with the person doing the requirements.