I've been a long time user of NetNewsWire -- the best desktop news aggregator app on any platform. But I'm switching to Google Reader.
Reason number one is portability. Forget synching...it just doesn't work reliably, and I don't like NewsGator's online version at all. I'm now on my Nokia E61 phone a lot, and the mobile version of Google Reader works great
Reason number two is that I want to share. My delicious postings are one of the main ways I've been sharing lightly -- light blog posting in general, except for brief spurts like today. I think that Google Reader will make it easier for me to share. For example, here are all the postings I've gathered in my "bryght" folder. It's not ideal -- I can only tag, star, or share items, without leaving any commentary as I do in delicious -- but I expect it will get better.
The big downside of Google Reader? No authenticated feeds. I have many beefs with Google and the way they're dealing (or not dealing, as the reality is) with identity: this lack of support for authenticated feeds seems another aspect of this.
So, there you have it: the best feed reading experience that synchs with your mobile is currently Google Reader.