Hey All,
I was just curious if those who have the upgrade are happy with all it has to offer. I have been contemplating whether or not to purchase it and would love some feedback before I spend the money and buy the upgrade for the 110. What are your thoughts? Are the maps any different or any changes noticed... I know it makes your 110 into a 120 and thats great but what is everyones overall opinion of it? Was it worth it? Thanks for your guys time and I look forward to your input... 
I just upgraded over the weekend and am extremely happy that I did. At first I wasn't sure it would be worth it, but the improvements to the UI are worth the price alone. Here are a quick few bullet points:
Source navigation - So simple, yet such an improvement. Just having the sources on the left side whenever you have an audio source open makes switching between sources a breeze.
Ipod/phone controls - A million light years better. You can actually toggle from shuffle from the main screen (finally!). I like being able to navigate a playlist from the scroll wheel. Just about everything about it is better. My only complaint is I haven't found an easy way to go directly to playlist from the main screen. I could be missing the shortcut though.
Pandora - Wasn't sure how good this would be, but it really is pretty slick. Right now the app can't run in the background, but given the not infrequent updates to the AVIC Feeds iphone app, I think this will be updated eventually. Sound quality is good (better than XM) and haven't had any buffering issues, and I have it on High Quality
XM - This is the biggest improvement IMO. I only go to my handful of favorites, and now they are all right on the right scroll wheel. Very easy to navigate. And I could be crazy but the sound quality sounds a bit better to me as well.
HD Radio - No huge improvements, but again, having your shortcuts on the wheel is great, even though they STILL don't list the station number.
Maps & POI - Honestly haevn't noticed any real difference. The graphics are a bit spruced up and look a touch prettier, but that's about it.
ECO Driving - Don't know what the heck this thing is supposed to be for. I just ignore it.
Musicsphere - Reading some dodgy reviews of how this works on PC/Macs and takes forever to scan your library, I haven't even bothered to use it.
Bug fixes - I was having serious problems with my new Iphone 4.0 in that it was failing to sync all my contacts which obviously was a huge pain (esp. as it made my AVIC app useless). This bug was actually the reason I upgraded, and I'm happy to say that it works fine now.
So that's my summary of all the changes I can think of off the top of my head. I wouldn't hesitate to upgrade if I were you. Got mine off Amazon for about $170 I think. Honestly, if you have a 110 and got the $200 rebate, it's common sense to pay for the upgrade. you won't regret it.
Excellent review. Echos my sentiments.
h-f
I've had the upgrade for 2 weeks now. I would say that it is worth it. However, there is one thing that I do not like, but it is not a deal breaker. I kind of preferred the larger XM Radio logos on the older GUI. I had gone a long time with my AVIC-Z1 and when the channel/station logos changed, I had a lot of generic icons. When I finally bought my AVIC-Z110BT I got all of the new icons and loved them. Big, bright and colorful. Now, with the new 120 update, the XM logos are all teeny tiny postage stamp-sized logos. 
Aside from that, I love everything else. There seems to be more room for artists names and song titles, but there is still not scrolling of longer names.
Maps look the same to me, but the maps are spot on for me here in Maryland. I never had any complaints about the maps at all. I wish the maps could be displayed on the back headrest monitors. Right now the old AVIC-Z1 was better about that. On the Z1 I could display the navigation maps up front and in the back. I could have the iPod display in the front and back also. XM Radio could be displayed in the front and the back, TV in the front and the back, and DVD in the front and the back. Now, the only thing that shows in the back is TV and DVD. Maybe my installer hooked something up incorrectly, but I don't see how a newer model does less than an older model.
I also don't get the whole Echo Graph thing. Does the upgrade disc not give you everything that you get with the real 120? The manual shows screens that I can never find on Echo Graph.
Wow, amazing reviews guys!! Thanks for all the info... Great review tombonneau... I totally appreciate all that responded. You guys made my choice very simple. I will be purchasing the upgrade asap. Once again thank you for your very informative review and taking the time to respond.