How good of a search is it? Will it search through my forums topics etc? Is there a tool to see what the search results would be?
It's just like Google. If your site is well indexed in Google, then it will find everything, if not, it doesn't help.
The search you put on your site uses the same index as the main Google.com search engine. I.e. you can only find what Google has already indexed of your site. If your site is constantly spidered, this isn't a problem.
i think it's pretty good if you have sitemaps as well, but i find the design of the search box with sitesearch quite ugly though.