I have one of the
current Alesis units & years ago at another facility (in the days of records & cassette tapes), had a Thompson Vocal Eliminator. The results can be astoundingly good or purely awful & it all depends on the mix & the quality of the original. Your's usually in good shape with original CDs. Whatever the device finds that is common to both channels is what will get zapped. Typically, that includes lead vocals,
bass guitar & some portion of drums. That leaves whatever was mixed left or right of center & usually includes cymbals, synths & keyboards, rythym guitars, backup singers,
etc., in varying degrees & quality.
The biggest problem I've found is in lead vocals with lots of
reverb, which by nature, will be slightly left & right of center. That will most often kill or severely reduce the desired result. If your end plan is to produce a karaoke
track for someone to sing along, you can mostly
cover the remaining traces of lead with the karaoke singer, especially if you apply some
reverb to their vocal.
The Alesis Vocal Zapper has presets which allow for quick setup, but no flexibility in "tuning" to get better results. The Thompson VE was much more flexible in being able to change various things to get better results. Unfortunatley, the
current DSP-based model is quite expensive for varying quality results you might get. The Alesis is $99, so you've not blown a bunch of cash if it doesn't work every time. There's no way to predict how well it will work, you just have to give it a try with your material.
BTW, you'll end up with
mono end product as a result of the vocal reduction. The old Thompson
unit had a stereo synthesizer tp generate "pseudo" stereo & I'd guess the
current version does as well.