There were problems with the earlier ones. This is from the Wiki.
I see a lot of 350Z transmissions have been replaced
During the 2003 introductory year, many 350Z owners reported problems with the 6 speed manual transmission. The symptoms reported included: gear grinding, trouble shifting, hard shifts, getting locked out of gears, among other problems. This is something any potential buyer should be aware of when looking into a 2003 (and some 2004) 350Z model.
Why did the G35 not experience the same problems?
This is not an easily defined answer, but it is most likely due to the driving nature of the 350Z owners versus the G35 owners. The symptoms are much more evident under hard driving conditions such as fast shifting, or shifting at high engine RPM. With few G35 owners pursuing "spirited" driving, there is most likely a lower reporting rate of problem transmissions. The G35 and 350Z use the same transmissions, so the failure rate given all other variables the same should be even.
What are the failure signs?
Signs that your 350Z transmission is problematic: gear grinding and difficult shifting are the most obvious symptoms.
How can I fix the grinding problem?
Hopefully you find this problem before the 350Z drivetrain warranty expires. The typical fix is to replace the transmission with a newer revision. If you do not get it replaced under warranty, expect to pay $1600 or more for a new one (plus labor expenses).
It feels hard to shift sometimes
Transmission replacement cost
If you do not get it replaced under warranty, expect to pay $1600 or more for a new one (plus labor expenses).