One thing that strikes you immediately is the challenge of incorporating all the options available into the iPhone's limited UI space. The button panel at the bottom of the screen is a 3D object that you can spin round to reveal more controls.
And you need them, because there's a lot Starmap can show you. Not just the stars and constellations, but also planets, expected meteors and "deep sky" objects like galaxies, clusters and nebulae.
Features I particularly like include: the night-vision mode (tints everything red to minimise your iPhone's own light pollution); the fact that, by default, only objects you're likely to see are shown in lists of stars and planets (you can toggle this on and off if you'd rather see a list of everything, below the horizon or not); and the general feeling that a great deal of thought has gone into building the app from the outset.
The main issue with Starmap, and it's one you'll notice pretty quickly, is speed. Right now, the app does not use any accelerated graphics and subsequently, the frame rate is 3 FPS. Dragging the night sky around beneath your finger is slow and the animation jagged; as a result, it can be a little frustrating to use. Consider this before you buy.
This is a known issue, though, and work is under way on a fully optimised 3D version running at 20 FPS. It might be released as soon as October, we're told.
That (and one or two crashes) aside, Starmap is a fantastic educational app and very good value for the price ($11.99). Existing users should hold out for future updates -- if they deliver what's promised, Starmap will be an almost irresistible purchase.
Oh, and one other thing would be nice: a function that controls the appalling British summer weather, and clears away the almost permanent cloud cover so that we actually have a remote chance of seeing some stars.