What do you like best?
iMovie has been around since 1999, before Mac OS X. It's user-friendly, yet it contains some advanced features normally found on far more complex applications. For quickly editing simple videos, it's hard to go wrong with it. The user interface is intuitive and approachable, and yet it gives you access to powerful and precise tools.
What do you dislike?
Former versions supported dual-screen operation, with timeline and tool palettes on one display, and a full screen preview of the video being trimmed in another. The current version only uses a single display, which is a serious productivity hindrance especially for those using iMovie on a laptop. Its ability to remove hiss and hum from less-than-stellar audio could use beefing up.
Recommendations to others considering the product
If you're on a Mac, this will be your first video editor. Don't look down on it just because it's bundled with the operating system, it's really quite capable if you'll learn to use its basic features, and there's room to grow before you'll have to migrate to a more advanced NLE (nonlinear editor). The good news is that if you move to Final Cut Pro, you can import your iMovie projects directly into it.
What business problems are you solving with the product? What benefits have you realized?
I often use iMovie to edit short how-to videos of specific software functions, to quickly explain to customers what would otherwise take a lengthy textual description.