I got a 0-30 liquid gauge and liked it enough to start carrying them at TFTestkits.
They are 2.5" in dia. so quite a bit bigger than standard on a pool filter and the gradations are much wider and easier to read.
There big advantage is they will not freeze down to around -20 F. That's more than I'll ever see so I am able to leave my gauge attached all winter.
At 10-11 dollars, they are not a steal UNLESS you were ordering chemistry anyway......that makes the shipping essentially free.
So, do you need one? Absolutely not but they are obviously nicer than the air-filled and noticeably easier to read.