I've been using the BF water calculator for the past about 20 brew sessions. For me, I like it the best of the tools, and will continue using it. Disclaimers:
1. I make efforts to minimize the adjustments I need to make with acid (such as starting with a water source that's already close to what I'm looking for; thinner mash when I need things to be less sensitive to change, etc).
2. I've never measured my mash pH with a meter. I put a certain amount of trust in the calculators, and in my knowledge of how to use them in a way that mitigates their ability to be very wrong; and I go by taste and all other measurements I can take to ensure that I'm in a proper envelope.
Hasn't failed me yet in any way that I can detect, I will however be investing in a pH meter one of these days.