"Really that good?" is a subjective opinion. But I do find Mail.app excellent for offline access and integration.
Offline access is obvious, you can move and store email to your HDD, access them without internet.
Just like everything else, integration is a big advantage in OS X. Mail interfaces with just about everything:
- stores/pull email address from Address Book (with sync with iPhoto/itouch)
- works with Time Machine, so TM backs up your deleted emails
- integrates with iCal, so you can generate appointments based on information in emails
- sync with Mail.app in iPhone/iTouch
- all emails indexed by spotlight for fast searches