When we saw GMail years ago when it came out, people were very excited to get it. And moment they got it they could practically start using it as their primary account and put it to full use. This meant people spent lot of time with the product and came to like it or hate it, mostly like it as it was ads free and pretty fast.
With Google Wave, whoever saw the demo was very excited about. But when they finally got their invite, all that excitement didn't translate to wow! or ah! moments when working with wave. Aside from the stability problems, I think the key problem is that unlike e-mail wave isn't a widespread standard protocol - this means you can't use wave with all your friends and family unless they also have wave - which they don't as of today.
So in some ways, this invite-based marketing is actually having negative effect in this case. Waiting for the invite builds up people's expectations and when they do get the invite they see these stability issues and can't really use the product to it's basic full potential which leads to I'm-not-so-impressed-with-wave feeling.
I hope Google will open up the flood gates and let lot of people register so that people can actually use wave as a day to day communication tool.