I've had my Samsung Blackjack II for a few months now. One of the things about this phone that excited me the most was the integrated GPS. I unlocked the GPS receiver (follow the instructions on that site verbatim to unlock your GPS*) to work outside of just TeleNav. Unfortunately though, I couldn't get Windows Live maps to detect the GPS. So I decided to download Google Maps for Mobile and started using it exclusively.
The one thing to keep in mind is that the Windows Live App has more than just Maps to offer. It's a full fledged 'search' application with Maps as one of the features.
A couple of weeks ago, I found the patience (and the time) to startup Windows Live for mobile again to see if I can figure out why exactly the GPS wouldn't work. Turns out, it was working just fine. Since, I've started using Windows Live Maps for Mobile and I thought I'd chart out my comparison between the 2 products. Overall, feature wise, Windows Live Maps for Mobile wins. User interface wise, Google Maps for Mobile wins.
In spite of all these features, I'd warn you to consider charging your phone while you're using the GPS. On average, I've noticed that my fully charged phone runs out of juice in about 45 minutes when I have the GPS turned on. Yikes!
I can't wait for the next version of these apps. I think they'll try to outdo what they have currently, which means we win!
PS: Please do not use your phone while you are actually driving.
* - Neither Microsoft nor I take responsibility for this patch on your phone. Use it at your own risk.