I use both the strips and a liquid test kit. The strips are accurate enough for most uses, they are fine for getting s ballpark figure and helping decide whether you need to run a full liquid test. I know people slate the strips but they are useful, quick and easy to use, they will certainly warn you if you have a spike in nitrates.
I used the liquid test more when I was setting up my tanks, now they are established I use the strips as a weekly check after I have changed the water - if I think there's a problem I use the liquid kit to establish a more accurate levels so I can fix them problem.
And if you are really short of cash the strips are better than no test at all.