I use fresh ginger and prefer to use it best. I agree with the comments above about the uniformity, and I have found that you need to look for large pieces of ginger that are plump with a clean bright color, as opposed to the small, shrivelled dried pieces - the larger the better! The one's I pick out are very uniform in appearance and I find that this uniformity passes along well into the finished beer. I will add that I use about 12 lbs of ginger a month so I have a pretty good feel for it now. I use it sparingly at WP and WP for about 20 minutes then another 10 minutes settling and whatever time it takes to run off. I am looking for a subtle ginger character with no heat. If, on occasion, the ginger is less than I want I will add a little fresh ginger tea after fermentation is finished.
I add 180 degree water to mine (chop it with knife first) and then pop in in microwave and wave it till it is close to boil then let sit for 10 minutes before pureeing in processor then add directly to kettle. I think that this process helps extract more flavor and kills molds and such present on skin. Also makes it super easy to puree.
Just going to give a little hint: Lime leaves go very nicely paired with giner using same technique mentioned above.