My favorite so far is Voice Actions (Pannous), but I've also tried Vlingo and Eva. Observations after several hours of playing around:
Voice Actions is amazingly accurate at recognizing my commands and questions, but it's a bit slower than Vlingo unless I'm on a speedy connection. It's also more "hands free" and can answer a much wider variety of questions - I've had a few uncanny linear conversations with it. Works best with a custom voice (I use SVOX "Grace" and "Victoria").
Vlingo has a nice TTS voice (slightly better than the SVOX voices) but often fails to answer rather basic questions (such as What's the weather like tomorrow?) that Voice Actions always nails. Both VA and Vlingo can launch some apps, but can't play selected songs for example, or reliably read text messages or email (yet).
Eva couldn't get past the setup screen on my EVO for some reason, so I gave up trying.