Well, as Tore Mogensen said in the video for this pedal, it is extremely simple. But you'd be hard pressed to call it weak.
Let's first talk about non-sonic things: the teeny tiny size is great for cramming it onto pedalboards, the knob turns nicely and stays put, the LED is crazy bright, the art is very handsome, and the casing is rock solid. Not much more to say than that, but there really isn't anything to complain about either. Overall just a really solid pedal, also the one knob setup allows for easy foot adjustment of the level.
Now to sound matters. One knob and an on/off switch are all you get to control the mammoth 20db of boost on tap with this little beast; wound all the way up this thing provides a monsterous amount of gain, way way way beyond anything you'd ever need. The effect is instantaneous when the switch is hit and the silent switch eliminates any pops or slowed effect triggering. Which leads me to probably the best part of the Spark mini: the selective latching/momentary switch: with the latching switch you can easily just tap on the pedal and boost yourself easily into solo range (I've heard running it in the FX loop produces the cleanest of clean boosts with zero additional distortion to your sound), or use the momentary switch for fills and lead work when playing in a mix. For example: at a gig we were playing a song with tons of stop time and fills, as it turns out the other guitarist went AWOL so I had to play both parts, so thanks to the super fast normal-boosted-normal level transition of the spark I was able to be both the lead and the rhythm player and keep the dynamic shifts we had with the dual guitar setup (MAJOR lifesaver).
Now while this pedal is fantastic and a complete steal, there are some things I'd really love to see change in the future: the first is to have a little switch for color control, for example you can make the boosted signal slightly brighter or darker, or keep the coloration switch bypassed, as I have found that it really exaggerates the treble response on my amp, and maybe it's just my gear but at higher levels of preamp volume it creates this really grainy and screechy gain structure, with only a little boost this doesn't happen but as you go up this becomes more apparent.
All in all this is an awesome pedal and a great addition to any pedalboard.