I bought the Tom Tom app. I’ve got a 3GS and my wife has my old 3G. I didn’t really have a great desire for this, but my wife wanted the voice navigation and considering $99 got us two portable voice navigation systems, I thought it would be fun to try. I’ve never used a standalone GPS so I can’t really compare it. So far, I’ve been impressed and the voice (aux in over speakers) feature has been very useful.
It won us over one stormy night heading into Toronto for a concert. We were late and the highway was a mess. Since we had the GPS had the guts to venture onto the back roads and ended up saving a least 45 minutes. Other than the tail end of that trip I haven’t had a chance to measure performance amongst tall buildings. It appeared to have no issue even with the device sitting in a less that optimal location since I don’t have the mounting kit. Given that it seems to be working fine (and the fact I’m cheap), I don’t think I plan on buying that mounting kit unless if the price comes down.
My only complaint is that it never recognizes addresses pulled from my contacts, it always says they “can’t be understood - would you like to enter the address yourself?”.