I spent some good quality time with this game today, and was able to get through the Trench Run on the Easy level, but the Medium, Hard, and Jedi levels remain. I haven't tried them yet, but I look forward to the challenge.
If you don't feel like running a Mission, there's an Arcade Mode in which you run the trench or have a practice dogfight where you try to shoot down Tie Fighters, including the one flown by Darth Vader. Here you earn extra points for what you blow up, although the game also tracks accuracy. That's pretty amusing, because I just keep my finger on the fire button.
The dogfight was confusing at first because there are no enemies on screen. You see several angle brackets (which look like "L" and a backwards "L") and steer towards them. These brackets are "targeting locks" on TIE Fighters, which maddeningly strive to remain off-screen. This is especially annoying in Arcade Mode where you can spend most of your time trying to get something on-screen to shoot at. When you do, press the bottom-left side of the screen for a temporary slow motion mode which makes it much easier to aim. Darth Vader takes something like 30 hits to blow up, so don't be surprised if you run out of time.
There are settings available for Tips, Sound FX, Music, Invert Pitch, and Difficulty. There's also a Sensitivity level which you can adjust from 0-100 (it defaults to 40). I set mine to 20, and then to 50. It's probably going to take awhile to figure out the right balance. Game play is interspersed with clips from the movie (to skip, tap the screen), as well as familiar audio clips.
Important note: I tested this on an iPhone 3GS. Apparently, the game plays very slowly on first-generation iPod touch or original iPhones (even the iPhone 3G, according to some). This is likely to be the case for many intensive games of this type.
Minor nit: the game is labeled "Star Wars: Trench Run" but all that you can see of that name is "Star..ch Run". I'm surprised they didn't just called it "Trench Run" since the icon makes it clear that this is a Star Wars game... but that truly is a minor UI point.
All in all, it's a great way to spend US$5.00, and it would even make a fun stocking stuffer for an iPhone-owning friend or relative.
UPDATE: If the app freezes on you, press the home button and then relaunch. The app has only locked up on me twice, otherwise it has been very stable on my 3GS. YMMV.