Post code truncation is not a big deal. A few taps get you to a full address entry, as it finds town and street names on the fly. A couple of letters of each will usually get you to a list that you can tap. I figure that's no more taps than a full post code. Even a full postcode would still only get you to a section of a road, and you still require the house number for a complete location.
I have to disagree with you there, I have owned 3 SatNavs over the last few years and the first one was running Navigon software with the 5 digit post code search and it was the most frustrating thing ever. For example try entering a Pub that has no building number but is on a 5 mile stretch of an A road. With a full post code search the navigator gets you close enough that you can see to place. With the stupid 5 digit post code you have to drive up and down 5 miles of road to find the place you are looking for. Not only that sometimes the road names in the database do not match with the postcode. So for example the address might be 50 old high street but the navigator has no old high street for the first 5 digits of the postcode but instead lists another dozen or so road names that you have no idea if one of them is an alternative name. Totally frustrating and utterly useless at times.
The second Navigator I bought just recently was a Navigon branded navigator, full post code search was a real improvement but the software was just a real pain, it felt really clunky and the routing was worse than the old one, especially annoying was that it would tell me to turn left in the middle of a main road where there was no turning but there was a slight left hand curve in the road where as it would fail to tell me to turn where there was a real turning so I'd have to look at the map. I literally dumped that one after 8 weeks it was so bad.
Having tried two Navigon based navigators that were bought because they were cheaper than Garmin and Tom Tom I decided to splash out and buy a Tom Tom and it is miles ahead of the Navigon, everything works smoothly, well thought out interface, brilliant post code lookup and points of interest. No stupid error messages when trying to save locations, a real dream in comparison, really worlds apart. I recently bought a navigator on behalf of my father-in-law and as there was an offer on a Garmin I went for that. Not quite as nice as the TomTom software but again still a lot better than Navigon.
I must admit that the idea of having SatNav on my iPhone is appealing but after being so disappointed with Navigon before I decided I wouldn't waste my money even at the introductory price of £37. I decided it was a waste of money because there is no gaurantee that they will update it for proper post code search and the reviews on the app store say that the map data is old and generally the app is not that good.
My advice, don't bother, wait for Tom Tom even if it is significantly more than the Navigon software it will be well worth the extra.