It all depends on what you are trying to get out of it ...
If you want a precise measurement of the exact flow rate through your plumbing, NO ... the meters (in my experience) are only good for +/- 10% ... and even then, only if they've been properly installed (at least 10x pipe diameter downstream from any fittings or other devices).
If it is accuracy you want, there is no substitute for the good-ole bucket test.
However, if you want a "relative" indication of the current system to monitor for "upset conditions" (blocked intake, accidentally hit a valve handle, mechanical failure, etc.) then YES - they are great tool for this.
I use them on my system for this purpose ... I take the readings they give with a grain of salt, but look more closely how the readings have trended over time as a means to detecting if anything has changed and of the flow rate through the UV is still approximately "in the range" stated by the manufacturer.
Neptune provides a very good set of instructions on how to properly install and use these meters ... the problem is most people don't read them all the way through ... but if you follow them completely, these meters will be very reliable for many years without maintenance. Just don't interpret the numbers they give as gospel.