For example, asking Evi, "What's a good recipe for chocolate mousse?" yields specific recipes. Ask Siri the same question, and you'll receive a suggestion to search the Web. Similarly, if you ask Evi, "When is the next national holiday?" the app will respond with Monday, February 20, which is Presidents' Day. Siri pulls a result from computational knowledge service Wolfram Alpha that doesn't make much sense.

Evi also understands what you like and don't like, if you're willing to share. If you think one of Evi's answers is particularly good or bad, you can let it know by giving a thumbs-up or thumbs-down.

And while Siri allows only voice input, you can ask questions aloud or type them to Evi (both respond aloud and with on-screen text). On the iPhone, Evi, like Siri, uses technology from Nuance Communications to power the speech-recognition function, so True Knowledge charges 99 cents to cover its licensing costs. The Android version uses Google's own speech recognition capabilities, so that app is free.

Nova Spivack, founder of semantic Web service Twine and CEO of social-media aggregation service Bottlenose, expects a smart-phone maker to license Evi for inclusion on its own handsets.

"I'm sure they're desperately hunting for a Siri killer," he says.

Unfortunately, many would-be users haven't been able to experience it themselves yet. Since its Monday release, Evi's servers have been inundated with traffic as tens of thousands of people have downloaded the app and attempted to pose their own queries. The app is currently only available in the U.S. and U.K.

Trying to ask Evi a question throughout much of the week usually elicited a response such as, "I'm on it" or "I'm working on it," followed by "I'm having trouble getting a response from my servers. You might want to try again in a minute."

This has caused a backlash from frustrated users, who have given it negative reviews on Apple's App Store and Google's Android Market.

The team behind Evi is working on it, though. Tunstall-Pedoe says the company has already added "huge" amounts of server capacity to handle the app's popularity, and continues to do so.

And he's confident that, once people are able to use it, they will like it. "At the heart, she understands what you're saying," Tunstall-Pedoe says.