I've been a VoiceXML developer since 1999, when Nuance provided a free developer's version of their voice platform.  I'm experienced with VUI design, grammar building, speech tuning, etc.  Here's my opinion on some of the platforms for which I've done VoiceXML IVR programming. 

• Avaya has a couple of IVR platforms, and their Dialog Designer toolkit integrates with all of their telecom components. Their Voice Portal IVR platform is good, and it has a great team behind it.
• Nuance has a comprehensive speech recognition platform.  I really like the reporting.
• Genesys bought the VoiceGenie platform.  I haven't had a chance to use the GVP platform, but I hear it's very similar to the VoiceGenie one.  I was impressed at how well designed this platform is for large-scale implementations.
• BeVocal is great, and provides plenty of logs, documentation, etc.  I particularly like their Vocal Player section, where you can download a packaged file containing the utts, prompts, tts and logs for any call.  The log looks VERY similar to the original Nuance Voice Web Browser logs.
• Visibridge is a simple, stable and lightweight VoiceXML browser that uses SAPI and TAPI.  You can develop and test applications on your laptop, and deploy them on systems that are using VOIP, T1s, analog lines, etc.  The time-trial download is free and you can always grab a new copy when your trial runs out!
• Envox has a great VoiceXML browser that's tightly integrated to their existing platform.  I've used their system on a number of projects, and their extensions into their existing platform provides fantastic flexibility for IVRs.