I built a site a few years ago and used Justin Tadlock's Members plugin to set up roles and accesses for certain parts of my client's site. It's way out of date and they want a new system.

The client needs to be able to:

1) create content for specific users, which are companies. One set of pages for each user, and right now they have about 30 users. They need to be able to easily add new users and add new pages that are restricted to specific user.

2) have some way of controlling the display of pages and subpages in a menu, so that the logged-in user will be able to see only his/her restricted pages/subpages like in a widget.

The logged-in admin needs to be able to see a menu showing all restricted pages/subpages.

Is Role Scoper a good fit for my client's site? Or is there something else that might be a better choice?

Yes, that is all normal functionality for Role Scoper.

RS version 1.2.7 does not filter links displayed in the nav menus created in WP 3.0 via Appearance > Menu, but the newer release candidate code does. Try Role Scoper 1.3.RC2.