I'm certified in the Televantage, but have never installed one outside of class.  I primarily deal with Nortel Norstar and Avaya Partner.  I was very impressed with the features available with the Televantage product.

Be aware, I am biased. I have been involved in sales and support of Altigen product since 1997.  Technically reliable, excellent feature list from day one, fast and easy to implement and simple admin interface and troubleshooting trace functionality that an admin needs. Altigen can work with goldmine.
Unfortunately, your specific requirements, such as call flow and processing plus existing equipment capabilities(read: what functionality you haven't considered you have now and expect to retain, or the boss's favorite feature is not available)have not been described in your post and will greatly impact whether YOU think this equipment stinks or is the best thing since sliced bread! Correctly defining your expectations will help you build a check list of requirements to choose your vendor, have them sign off on and hold them accountable for providing what they promised! Have them bring a demo box that you can configure with them, or visit the vendors office to do it as this is a large investment. Check out financials of both companies.