I'm a new 2008 Hybrid owner, too!
I pretty much finished installing the GMX550 on my tahoe this morning. I took my time so it took about 2 hours. A lot of the time was spent removing the center console trim without damaging it. I think the hardest parts of the install are the removal of the trim and the routing of the Lockpick harness.
Testing for me is in the early stages. I haven't Lockpicked the factory rear view camera yet. I have tested the ipod connection and it seems to work fine. I'll be driving to L.A. later today so I will try the DVD and Nav override features then.
One tip. The Gen6 setup (not sure how it compares to older versions) takes up quite a bit of space. I couldn't fit it behind the factory stereo so I put it all in the glove compartment, which now has very little space left over. If you choose to do this, you need to push the entire Lockpick harness back through the hole that the factory connectors come through. From there you can then route the part of the harness that goes to the Lockpick box to the glove compartment. I figured this arrangement will also make future additions or USB upgrades easier (I won't have to remove the factory stereo unit again).
Hope that helps.
I'll give another update once I've tested out the other features.
---------- Post added at 09:07 AM ---------- Previous post was at 09:02 AM ----------
By the way, my next project is to add Bluetooth capability. I'm leaning towards Scosche's BlueFusion.