The phosphate and alkalinity meters get reasonable reviews, although there are problems with batches of reagents. The calcium meter seems a bit more iffy. The sample solution needs to be dilute with 10 parts RO/DI or distilled water before running the test, which might be an accuracy issue. In any case, the calcium meter seems to be more error-prone.
I've always used kits, mostly Salifert, IO, and Hach, personally.