I THINK the GPS is separate (as in: you don't get both for $150!). You have to buy their other GPS. My expectations of negatives are:
A. You can't view both GPS and your data at the same time.
B. Garmin will not be dedicated to providing the best and most in depth ODBII info, because that's not what they do. It'll all be targeted at your average consumer. Thus, some features may be lacking compared to Scangauge.
C. Wouldn't hold my breath on upgrades, improvements, software mods, etc, whereas Scangauge is always working on improving, because that IS what they do. For a living. My only bummer on Scangauge is that I'd like an integrated method for including exhaust gas temps by adding a sensor kit. But lord knows Garmin won't offer that.
And, of course, there are other ways to go. Let's say you get one of the new Google phones or android phones. And you use that for your GPS. Well, you can get Droid-based bluetooth driven ODBII wireless viewer software and a bluetooth ODBII transmitter (for like $60). Then your phone is your GPS is your ODBII gauge is your MP3 player. Which would be sweet, if you didn't have to be your own personal tech support to keep all the crap working. If it was that simple, people wouldn't actually buy Garmin units anymore.