The MPP (Massively Parallel Processing) architecture is very strong feature of this appliance which helps in running ugly time consuming warehouse report queries very fast - These would have taken forever to run on other RDBMS databases. The key is the fact that most of the unwanted data is filtered (projection and restriction) as close to the disk as possible. Standard SQL queries can be used with the appliance which makes it easier for porting existing applications and avoid a new learning curve in that front.
If the best practices are followed in terms of distribution keys, data-types etc. you can get real good performance boost.
Since this is an appliance, it has many hardware components inside and we have experienced lots of sporadic hardware failures - we are then at the mercy of IBM to replace them (Scheduling, webX, field Engg) which really reflects on the SLA. Many companies have complained about the Moose disks (bad batch of disks) which had to be completely replaced. For small shops this could be really inconvenient.
There aren't enough active user community for Netezza and so it gets very difficult sometimes. You again would need to open a ticket with IBM and get understanding of what it is (eg - SEU single bit vs Multi bit error) and get some documentation on the issues.
Also dislike the fact that the catalog backup pauses the system. (Understand that the catalog is in the host file system) But if IBM can allow a hot catalog backup, that would be very useful.