I'm using a cheapo right now but a decent one is on the slate this spring. I've got the 3-solution calibration setup (6.68, 4.00, 9.1
...and I've just used regular water to keep it wet. It's been fine, but I've been recalibrating every 2 weeks.
I did recently skip a few extra weeks and found that my well-water had changed it's pH downward, which may have been a seasonal thing. I recalibrated and found I needed to pH up to 6.5 where during the summer I'd ended up right around 7pH and had to bump it down.