Are test strips accurate?

In my experience they're fairly accurate.

I wouldn't call them accurate exactly.. They'll give you a ballpark number.
Their margin of error is something like +- 20 ppm.

On my tank, it was way off. I actually have older posts saying I had extremely hard water. Its actually softer by more than 80 ppm

Sent from my Ally

They work ok for pH, but not on the others. The ammonia, nitrite and nitrate strips are notoriously inaccurate. I think everyone tries them first because they seem easier, but eventually move on to liquid test kits. Also, the longer the bottle of strips has been open, the less accurate they get (in my limited experience).

They also have a very short shelf life and once opened deteriorate very quickly.

+1 on test strips being finicky to work with.

I use the dip strips just at the start of the cycle but then I move to drip tests the closer it gets to the end of the cycle. I would have to say that they are ok for a general ballpark guess but other than that, thats it.

In my experience they suck big time. The pH part expires pretty quickly and will only show very acidic or very basic (vinegar to bleech), the KH read low, GH too high, and all of the nitrogenous wastes have never shown up at all on any of the different batches of strips I have had.
You never know how long they have been out of the factory when you buy them so unless you KNOW they are fresh, they probably won't be very accurate.
I returned some fish to Petco and they said my params were bad according to their test strips so I showed him my params according to a liquid test kit, had the guy test their own tanks, and they read even worse. He gave me new fish.