While in the UK, I had a few minutes in Hobbycraft and Rymans, neither of which had the masking fluid I wanted to try, so it came courtesy of Amazon Prime (thanks to our younger son!)
Back home, I have had a chance to try it out and I am pleased with it. Unlike it's look in promotional pics, it is white in the bottle but when applied and dried, it is clear. This makes it great for use on all colours of paper or card (disclaimer here - masking fluids of all types may not work well on soft seized paper).
To use it - don't shake just dip your brush in the liquid and paint onto a DRY surface. Allow to dry or speed it up GENTLY with a dryer, before removing. I use either a finger or a soft eraser. This is definitely going to be my main masking fluid. Coloured ones often stain and although you can get away with that often in a water colour painting, in card making, it's more of an issue.
My trip to Rymans, was actually for something they didn't have but I couldn't pass on a quick browse and ended up with two types of white pen. I have the usual fine paint type markers but they do have a tendency to get messy or stop flowing. I'd heard good reports of the Signo but nothing of the Pentel one, so got both (under £3 each).
My favourite is the Signo by Uni-ball. Don't get me wrong, both write with fluidity but the Pentel is not as opaque, chalkier looking. I suspect, but have not tried, that with the Uni -ball, you could also heat emboss. Both take a little drying time but the Uni-ball is bright opaque white, which is what I usually want.
So there you are, my honest opinion. Unsponsored as usual!