It is great when you want to use it on a given query. It makes great suggestions very quikly.
It is slightly painful when you want to tune a stored procedure as a whole. Especially when it is large and can take several hours to run. I am trying to find what you actually need to put into a trace so that the index tuning wizard will accept it as a workfile. The documentation is useful as ever – it will only tell you what you need to know if you ask exactly the right question. Anything else will drown you in useless details of other features.