Depends on how you plan to deploy. I have 6.0 at home, but remain on 2000 at work. I know they have worked out the nastiest of the issues with 6.0, but the ones who have to rely on synchronization are staying with 6.0. Apparently there is still an issue with synching and Sales Opportunities although I understand the Outlook issues have been largely resolved.
If you are a standalone or network based multi-user, 6.0 has a lot to like. But 5.x is pretty rock-solid and seems to get the job done for most.
The next major release is supposed to be a complete re-write in SQL, but they have been promising something like that for years!
HTH
swk