None (ok, navigon seems well received too - we have both and we both prefer TomTom) of the other paid nav applications come anywhere near close to TomTom, TomTom just looks, works and behaves how you expect an iPhone application to.

I really detest the applications which use their own UI, take for instance "sygic aura" - the map view is lovely, it's accelerated 3d and it's super quick...however, once you go into a menu you then realise how awful it actually is.

You pays your money and takes your choice, however, IMHO the smart money is on TomTom.