Here the result of a little benchmark I made while 2005's Christmas. Note zfs is more or less still under development.

Version Solaris Express build 27
Platform x86
Processor AMD Athlon (at 1200Mhz)
Memory 512MB
Disk One IDE disk
Storage One slice dedicated
Tool Bonnie++ version 1.03

I use exactly the same physical region for both ZFS, UFS. Though, while I made test with ZFS, I had more data (not sure there is an impact).

read-write.jpg random-seek.jpg sequential-create.jpg

As you can see, UFS is better than ZFS in read/write operation, while ZFS have really better result in create operations.

The whole result of bonnie++ benchmark is available here.