When I tried Evi some weeks ago it was a complete disaster. In a week of testing I could not connect once because of server issues. The developers said they were adding server capacity, and it seems they have. In extensive tests of the app this week, I only had one time when Evi was busy and couldn't respond. That is a great improvement.
I found a lot to like now that the app is working, and some things that Evi excels at. When I asked Evi who the Chancellor of Germany was, I got the correct information, as well as pictures. Siri also got the question right, but no pictures. I thought the Evi experience was a bit better.
When I asked for the best hamburger in Omaha, I got referred to a web site about Omaha Hamburger Joints from Urban Spoon. Siri rated the best ones based on reviews from Yelp. I thought Siri was more helpful, but Evi does include a web browser so you can stay in the app. Sometimes Evi does figure out the best local favorites without sending you to the web, but it seems hit and miss.
For current weather Evi is second rate. When I asked for the weather in Phoenix it suggested the National Weather Service Web Page. Siri answered me directly and brought up the iPhone weather app.
Where Evi really shines is if you ask a question with rather complicated syntax. Asking "Who is older, Barack Obama or Michelle Obama", Evi parses it all and gets it right answering the question directly. Siri gets you a Wolfram Alpha page with information, and you can scroll down to see the relative ages yourself (the President is 50, while FLOTUS turned 48 last month). Evi wins that round on presentation.
Both apps use Nuance speech recognition, and it is excellent on both Evi and Siri. Evi still has a lot of negative reviews on the App Store because it simply didn't work for weeks. Now that it's pretty much up and running, reviews are getting more positive.
If you have an older iPhone that can run iOS4 or greater, Evi is now a worthwhile investment. It can't interact with your calendar, use reminders, or wake you up, but it is very good at answering questions. If you have an iPhone 4S, Evi is probably worth the $0.99 because it does do some things better than Siri.
Of course both apps are early in their development. Siri is still called a beta by Apple, and Siri has also had some server issues rendering it unusable at times. I'm glad I took a second look at Evi, and suggest you do the same. Check the galleries for some direct Evi-Siri comparisons.