Wave is a nice platform to program for. I have never written any gadgets but I have played around quite a bit with writing robots (and I wrote a DevX article on robots a while ago).

I still think that the Wave web app itself can stand improvement, but I am enthusiastic about the platform. The really exciting time will be when an open source version can be easily installed on any server for private Wave access inside an organization, a family, etc. (and still support federation to Google and other Wave providers).