A couple of things on the review first of all (I have had a 540 for a few months now):-
The 540 does not have MP3 playback ability, that gem is reserved for the 940 unless I'm mistaken.
The map scrolling on the 540 sometimes suffers from the same problem of sometimes interpreting a press as a scroll, although it's true that the menu system doesn't suffer this.
Whilst the 540's traffic will direct you to smaller roads, the "HD" traffic coverage is much better on these roads than it's predecessor, which AFAICT only coverd motorways. It is far from perfect, and seems to have had some teething troubles, but is improving - just today, for example, it reported a queue on the A1081 into St. Albans from the north - TT's "plus" traffic would never have done that, I'm sure.
The 540 will also navigate to Latitute/Longitude, inbuilt (or downloaded or maually added) "Points of Interest" or even the location of your "Buddies" as long as you both use the "Buddies" service, as well as the usual recent destinations, favourites, home, or current location plus post code, address, and 'city centre'.
The 540 is TT's top-end *UK* edition (I guess the same applies to the NavMan). The others in the x40 range only include different maps# - at something like 50% more cost (the 940, IIRC) for little more than maps of a different continent, it's not really the sort of thing a 'typical' UK based user would go for.
# acutally, the 940 - europe, US/Canada maps - also includes MP3 playback, FM transmitter, and inertial navigation for use in tunnels &c, but otherwise, unless i've missed anything, they are essentially the same device