The actual quality of the effect produced is great. The EM has a somewhat unique tone and can double as a chorus. Rumour has it that this is the 'chorus' pedal favoured by Andy Summers in his stint with the Police, and with the Range past noon and the Color down it's easy to coax chorus like tones from this box. It can also do standard flanger sweeps but they're not as intense as some other units I've tried (eg MXR flanger). Compared to the deluxe electric mistress this battery version is supposedly more hi-band flanging with greater swirl.
The catch is, the standard Mistress is noisy as hell and suffers from the worst volume drop I've ever heard on a pedal. Rolling the Range past noon would bring on a sea of hiss at the bottom of the LFO that sounded like the surf crashing upon the beach and was really noticeable whilst playing. This really limited the useable sounds you could get out of the unit. When turn on, the pedal would cut you volume about in half. Mine was an earlier unit (not a blue one, but with no dashes in the circles around the knobs). It's possible it was a lemon or else later models improved somewhat, but at the cost these are going for (the Gilmour/Summers connection means they regularly sell for US$150-200) you're better investing your money in something else, unless you're comfortable changing all the resistors and caps to lower noise ones. The current reissue Deluxe Electric Mistress will get you close and doesn't have the noise and volume issues. Howard Davis can even mod it to the battery Mistress tone if you want.
A lot of cool tones ruined by noise and volume drop.
A lot of people complain about EHX's build quality, but there's a lot of near 30+ year old pedals still floating around in good working condition.
Unless mine was a lemon, this pedal is more a vintage curiosity than a modern workhorse flanger. The tone of the effect is great, but the volume drop makes it useless for live performances unless you boost the input, and the noise makes it virtually useless for recording. For under $100 you can just laugh it off, but I paid $150 for mine and I've seen them sell for $200 regularly. Unless you're swimming in funds I'd suggest you invest the money into an Deluxe Electric Mistress, ADA Flanger, MXR Flanger or the FoxRox TZF instead. They don't have the same tone as the battery Mistress, but they'll be far more useable.