I would like to contribute to this thread from the perspective of a hardware store manager

... We use a Waterlink Spinlab from LaMotte. I've been pretty happy with it, but there have been times when I would test a sample twice in a row and get drastically different results. But I know when the results are off, because I talk to my customers and ask them what they've done recently, think about weather, look at previous results, etc. To me, as a hardware store manager, it's important to:
- know which equipment works best (and/or what you can afford), and the limitations of what you do have
- know how to recognize faults in the readings
- spend a lot of time lurking on TFP learning from all the vast experience and expertise, rather than relying on the chemical sales rep's biased information
- teach the homeowner about pool chemistry basics, and give them the power to learn more by sending them to TFP
- use control samples to compare results periodically to physical strip tests and other stores to check whether calibration might be needed
- not be a jerk and just hand people the recommendations on the sheet without explaining, crossing things out, explaining timing, etc.
To homeowners and pool managers: It's ok to go to the pool store if you know what YOU'RE doing, so you can tell if the person you're buying from knows what SHE'S/HE'S doing and isn't just trying to sell you chemicals, and
if you're willing to pay a premium for things you can buy at a cheaper price at grocery/hardware stores. Any test can be off, especially if you're inconsistent about taking samples. I had a customer that was getting high CYA readings and was about to drain water off their pool, but it turned out they were taking samples from the very top of the pool!
In conclusion, I want to reiterate that
I am selling you chemicals, and I can confirm that we are biased sources of information. Our information comes from our sales reps and blanket coverage recommendations from testing equipment companies, and even if you get a knowledgeable "veteran", there's still a good chance they will be biased. Learn how to check us!