The Spark Mini is a great clean boost, without any perceivable sound colouring or noise. The build is solid, the knob is comfortable and the switch is soft (without that clunk that most oldschool switches have), so it doesn't hurt when you step on it only wearing socks. Works fine both as a boost before my preamp to raise the gain, and in the FX loop as a volume boost. Comes with self-adhesing rubber feet and a tc electronic sticker. Looks quite cool, too.
The switch also has a hold feature (tc electronic call it PrimeTime), meaning that if you press and release the switch it works as usual, but if you hold the switch down for more than a moment the pedal switches off when you take your foot off. Very useful if you only want to accentuate a specific part.
The only downside is that battery isn't an option, but I don't know how you would do that in this size.
Great pedal in a small form factor and for a low price.