We used Zenith for over a year before moving on. As another reviewer mentioned Zenith is great for the new MSP to get your feet wet, but you probably want to move on once you get to the advanced level.

Strengths
CHEAP
Quick and easy setup
Minimal knowledge required to setup
Quick integration with Autotask
Back end NOC to handle things your team lacks experience in
Pay as you go: They begin billing or stop billing same day.
A great way to get alerted at 3 am to a problem.
NOC is there to fix things for you after hours.
Ability to push work like service pack installs to them

Weaknesses:
Confusing and difficult to tweak the monitoring settings
Manuals are sometimes hard to understand
Product support is horrible and from overseas
SNMP monitoring is useless
Only monitors BackupExec backups
Scripting is a real pain,requires a heavy investment in time
Patch management seemed to be spotty
NOC often would kick back simple tickets back to us for completion
NOC often would take hours or days to get to a problem ticket - we ended up doing the work ourselves to avoid breach of SLA.
Hard to understand NOC technicians

Overall, Zenith is a good place to start as a first RMM tool, especially if you do not have SQL or Exchange skills in-house. The product worked very well for basic alerting and monitoring and the included LogMeIn Pro made it even more affordable. Unless you have no plans on getting into scripting or really need their back end NOC support, I would suggest just starting with a full featured RMM like LPI that you can grow into.