Personally, as I have been selling the Shoretel phone system here in Utah, I have not come against Nortel as a competitor. This year they went bankrupt and I think this article is a good source of information and insight from Shoretel's CEO, John Combs. http://blog.shoretel.com/2009/01/nortel-demise-marks-beginning-of-huge.html
If you don't want to read the article the part I took out of it was that manufacturers that have grown up in the TDM world are having a difficult time migrating to the IP world. He suggests that Nortel may be the first, but it isn't the last to suffer because of their inability to adapt to the changing environment.
Right now you can trade in your old Nortel phones and get over $50 for them if you go through TAMCO leasing.