I'm setting up WPMU for a client's intranet. One of the key features they needed was the ability to create forms, for example, for running small surveys, getting feedback, etc.
Gravity Forms is fantastic for this and the training overhead is low, which really helps.
But we've hit something of a problem with Gravity Forms on MU.
1) On each MU blog or site you're asked for the registration key. It should be possible to enter this once, so long as it's a developer key, I guess.
2) If you click the uninstall Gravity Forms button on one blog, you'll deactivate the blog everywhere. That's dangerous and could cause issues, although it would seem that it doesn't actually delete forms and settings from other blogs.
3) Even bigger issue - only a site admin can access forms on a MU blog. It would be rather more useful if admins (and maybe editors) could access forms on a MU blog - this is when the plugin has been activated as a site-wide plugin.
Here's my installation settings for the development environment:
PHP Version 5.2.6
MySQL Version 5.0.67
WordPress Version 2.9-rc-1
Gravity Forms Version 1.3.8
That's WPMU 2.9-rc-1 by the way!
And just to add, I still think it's a very good plugin :-)