I've got this as a replacement for Boss Waza Air, since I really got tired of Boss' poorly made mobile app. Spark app is very good and has a lot of useful features. Plus my Waza Air sometimes had connectivity issues, when trying to connect transmitter with headphones (also it had obsolete micro USB as a charging port, yikes!).
Spark Neo resolves all of that - connection is very stable, app is superb. However, I still think that Waza sounded better, especially on mid and low-gain. Plus it had neat spacial audio feature, which Spark does not have.
I also liked that Boss had 5 presets available, while Neo has only 4. This thing also does not have individual volume controls for guitar/bluetooth audio, so you have to adjust volume of backing track using your phone.
Considering that it's MUCH cheaper than Boss Waza, I would rather go with Neo.