I use this on an Iphone 3 and it works for me. Its not perfect and it can't compete with a dedicated GPS but its still impressive for your phone + a £25 investment.
I had an Iphone4 for a few months and that was a marked improvement. The only downside to it is the short battery life. If in constant use then you only get about two hours.
Its a good complement to whatever existing GPS you have or as a back up to your old fashioned proper VFR nav but I wouldn't reccomend it as your sole means of navigation.