I've had a Z2 in my sedan for about a year, and recently purchased a F900BT for my Jeep...
First of all, I know some of these issues have been discussed on here in various threads, so I'm sorry if this is a bit redundant.
Before installing this unit I had considered it an "upgrade" from the Z2, due to its better screen and iPod controls. In my opinion, it is definitely not an upgrade from the Z2. Here's why:
1) The startup time is ridiculous. The unit is barely usable (if at all) for the first minute it is powered on. This gets worse with the iPod connected. There is a considerable lag with button presses until long after the "system is starting up" message is gone. About all you can do is adjust the volume of the currently playing song or station. You cannot change the song or station for at least 45 seconds.
2) The screen is not nearly as bright as the Z2, and can also not be dimmed as much as the Z2 for night viewing. The background changes to night mode at a specific time, rather than when you turn on the headlights.
3) At low volume, the quality of the voice prompts is terrible. Very static/digitized sounding.
4) There does not appear to be a way to change the map view to "track up" instead of "north up" when in 2D mode.
5) There's no easy way to mute the nav guidance voice only, from the map screen.
6) The navigation system cannot be controlled via voice command. You cannot speak addresses or POIs.
7) You can't click and hover over traffic incidents to get more details about them.
8 ) No "dots" on freeways showing the exact speed of traffic.
9) The menu system doesn't make nearly as much sense as the Z2. Some settings are buried several levels deep, and other less important items are right on top.
10) There's no off button! With the Z2, you could hold the AV button to turn off the radio. With this, you can hold the volume knob for mute, but it mutes even the nav guidance.
11) There's apparently no way to turn off the beeps when a button is pressed, without turning off all system sounds (including nav guidance and voice prompts).
12) The voice prompts when using the voice command are very unnatural.
13) The iPod requires the use of the AV1 input for both audio and video. The Z2 was able to digitally transfer audio from the iPod (with the iPod adapter).
There are some positives to the unit, but they do not nearly outweigh the negatives, in my opinion:
1) The screen resolution is fantastic. However, as a result many items are simply too small to be easily seen from the driver seat.
2) I like that there is a volume knob, rather than the up/down buttons of the Z2. However it is a bit too shallow to be comfortable to use.
3) The iPod controls are vastly improved from the Z2 (they are the same as the D3). The speed is also much improved, and it supports video.
4) Bluetooth is faster and has better audio quality than the Z2. You can now also adjust the call volume while on the call (something I thought was ridiculous that you couldn't do on the Z2).
5) Dynamic volume adjustment based on speed for the nav guidance is very nice, and is long overdue.
Hopefully some of these issues will be addressed with a firmware update. In the mean time, I hope this helps others with their decision of whether or not to buy a F900BT. In short, I feel like I'm beta testing this product, and yet it has been released to the public.
Dave