Okay, just had a play. Sorry about the length of the "review".
First off, the level music is missing. This is unacceptable in a Duke game as far as I'm concerned. The music is one of the most important parts of the game atmosphere, and I'd trade off the CPU time needed for a really uber-smooth framerate (which it is, so good job there) for the music being available. I don't mind downloading an extra "music pack" or something in order to get this, even if it's an extra 600MB+ to avoid having a music sequencer in the game code.
Secondly, the controls. The analog controls aren't analog. They're a circle which a "Dead spot" in the middle, and they don't react anywhere near as fast as they should. I initially thought that it was impossible to change direction without lifting your finger off the virtual control pad, but I've realised this isn't true - it's just very laggy unless you lift your finger off the pad. The pad isn't analog as such - the movement speed isn't proportional to the distance from the centre of the pad, which makes adjusting aim as you go impossible. As the controls feel so laggy, this is almost a moot point. I would strongly suggest that the developers go play Wolf3d and see how this should be done. I also believe the control system isn't allowing the control pad to be "extended" if the user leaves their finger on the screen, so if you move a little bit too far off the virtual pad, the control input stops, and so does Duke. Again, this makes the controls utterly frustrating.
The digital controls don't have this problem, and they're very responsive. They just feel completely unwieldy to me - I gave up on using "keyboard-like" controls for FPSes a long time ago.
Tap-to-shoot on the screen is quite helpful, and seems to work reasonably well, but definitely doesn't make up for the general problems with the controls.
Onto the positives, the menu system is nice, the graphics are spot on - basically on a par with the XBox 360 port in terms of clarity and quality, and the frame rate is excellent even on an iPhone 3G.
There is no multiplayer.
Let me say that again. There. Is. No. Multiplayer. At all. No Dukematch makes Rolphus sad.
Overall, most of the technical aspects of the port seem to be pretty good, but it seems like the polar opposite of Wolf3d. Instead of focusing on a relatively unambitious project but nailing the controls and overall feel and response, it seems to me like these guys have taken on a fairly difficult project but not given it the control tweaking and polishing that it really deserves.
To me, this difference is about having someone intimately involved with the original project (i.e. John Carmack for Wolf3d) working to ensure it's the best it can be versus a (clearly very good) company contracted to port the game who has less of a link to the original game and less of a feel for how it needs to play.
In summary: If you buy Duke3d iPhone, be aware that you're buying a game that is in dire need of a control scheme fix to be playable, and music to be a complete Duke experience. If these are sorted out, I think the game should be a great single-player port. Add multiplayer, and the game will be amazing.