This is probably the best bang for the buck Envelope filter on the market. It tracks well and has quite a few tonal variations. The sound is very liquid and can imitate most popular Env. filters. The HP and BP filters are pretty useless for bass unless you Bi-amp. The only other con is the sweep is only in one direction, but then so is the MXR and it is twice as expensive and cant get to many tones the Micro Q-tron cant.
Electro Harmonix Micro Q-tron
Micro version of the Electro Harmonix Q-Tron envelope filter