Till now there is just a preview video explaining Bing's features, but on June 3 it will apparently replace Live search as Microsoft's new default engine. Google's success is mainly their extremely fast and mainly accurate search. Another point is the simplicity and clean layout. You just have this small search box and then the list of results without a lot of noise around, even the advertisement isn't that disturbing. This makes it much more usable.
It will be interesting to see the competition between this two engines. Google's share is impressing, if you just take the searches that reach my site which is mainly or just visited by developers. Obviously I have only a small share, but still, having an average visitor number of about 100 per day and if I take all the visits from search engines between 01/01/2007 to now I get the following distribution of search engines:
Let's see whether this will change ;)