Ah, this is easy.
- Great integration with Gmail contacts, Gmail.
- Good integration with Google Voice, Google Talk.
- A better version of Maps, with innovation at a faster pace.
- No need to connect it to a PC ever for synching.
- Ability to download apps without having to connect to a PC.
The ones above cover 80% of my Android. It's simply much better for what I do often, and not much (if any) worse for what I do rarely.