0.51 is working on completing its initial sync (about 1.1 GB), so I thought I'd post what I've seen so far vs. 0.38.
1. Java with 0.51 uses about 12% of CPU instead of the 99.5% of CPU 0.38 used.
2. I had to stop and restart 0.38 several times a day, and several times to complete the initial sync. Although 0.51 has yet to complete the initial sync, 0.38 had had to be stopped and restarted several times by this point during the initial sync.
3. I have some big (10MB+ attachments). 0.51's log frequently reports the following when syncing an email with a large attachment:
2007-07-28 13:12:48,203 WARN [Timer-Offline-Main]  HttpMethodBase - Going to buffer response body of large or unknown size. Using getResponseBodyAsStream instead is recommended.
4. Thanks for adding the Calendar!
As I see more things, I'll post.
All the best,