We all know ZFS is great. RAID-Z means fast RAID-5 without HW RAID controller. However the devil is in the details - while raid-z is great for writing speed, data integrity, etc. its read performance could be really bad if you issue many small random reads from many streams and your dataset is big enough that your cache hit ratio is really small. Sometimes the solution could be to make a pool with many raid-z groups - it means less available storage, but better performance (in terms of IO/s).
So if you want to use raid-z in your environment first carefully consider your workload and if many raid-z in one pool aren't good solution for you then use different raids offered by ZFS. Fortunately other raids in ZFS are NOT affected that way.
If you need more details then read Roch's blog entry on it.