I bought this pedal based on the fact that it was highly regarded, had good reviews, and that one of my musical heroes uses one. I was not disappointed.
The spring reverb effect was what I was really after, it offered me a very simple reverb sound that went beautifully with my clean tone. Given there is only one parameter knob on the Holy Grail, you can't tweak as much as you may like, but for a rhythm guitarist such as myself this didn't affect me too much.
The 'Flerb' setting is not something I had given a large amount of thought but after a play around with it I've found it to be quite an interesting sound. A nice addition to the pedal whilst again taking into consideration the price and size.
The only real criticism I would have of the Nano Holy Grail is the 'Hall' setting. When it's turned past around 3 o'clock, the delay between playing the chord and it coming out of the amplifier is large and noticeable. I am unsure whether this is a problem many people have as I have not looked into it but I feel it makes the 'Hall' setting relatively unusable.
The EHX Nano Holy Grail 'spring reverb' is definitely something I have kept in my pedal chain, even though I now own a Strymon Big Sky as well.