Most of the features of Final Cut Pro are here, including real-time effects, great colour-correction tools, and an almost-identical interface where you can edit in your own style. Depending on your needs, some of what's missing may not matter. For example, Final Cut Express does not offer either Final Cut Pro's OfflineRT feature or the ability to edit in formats other than digital video. However, other omissions will be missed: Final Cut Express doesn't come with a printed manual, and the program doesn't let you batch-capture footage.
The Final Cut Express editing interface is almost the same as that of Final Cut Pro. You're free to edit in a linear path and drag clips around the timeline. Like Final Cut Pro, you can have up to 99 video and audio tracks for full freedom in editing and compositing. You can even nest sequences (like in Final Cut Pro or Xpress DV) so that you can easily repeat certain edits. The biggest differences between the Final Cut Pro and Final Cut Express interfaces won't matter to new users; for example, creating a keyframe animation is different, but you wouldn't notice that unless you've used Final Cut Pro.
Capturing clips reveals the biggest differences between Final Cut Express and the rest of the editors; the batch-capture feature is notably absent in Final Cut Express. With batch-capture, you set the in and out points of each shot, name all of the shots, and then let your Mac capture all of the clips at once. This saves the intermediate delay of capturing the footage - which takes as long as each clip you capture - until the end, so you don't have to monitor the process. Final Cut Express lets you log only one clip at a time; you can set the in and out points and name the footage just like with the other editors, but you have to capture that clip immediately. The only other option is to capture footage without logging it first; afterwards you could manually divide and name the shots or have Final Cut Express automatically split them based on the timecode and date information.