The Battery life seems ok to me it uses a standard cr2032 battery.
The software is great, probably much more than anyone would ever need!! the manual for the pc sport software is 70+ pages and the help section is even more. I am still learning alot about the software and have even emailed VDO with some questions on a few things and they got right back to me and were very helpful.
I dont know much about the aftermarket services for the garmin, however the pc sport software allows you to upload your course data from a garmin unit and place lay out your course with mapping from google earth. This is something that I really think would be cool but after spending 250 on the z3 my wife would string me to a tree if I asked for a gps unit. if you want to see all of what the sofware has to it I can send you a pdf of the manual.
So far I love the unit the best feature that I have found is the ride profile chart that you can get after a ride.
I would say buy it, it also has a 5 year warranty and if you were to get it from performance bike they have a no questions asked return policy if you dont like it.
If I can figure out how to post a chart I will show you the ride profile option that I like so much