Being an UCLA Alum, I've watched a great deal of PAC-10 basketball over the years, and Derrick Williams is going to be very good.
The Danny Granger comparison is somewhat accurate in such a way that they are both very hard worker and show 'heart' for the game. Granger single handedly carried that New Mexico team to Sweet 16 (i think) while DWill 2.0 took Arizona to Elite 8. However, Granger's game relies a lot on coming off screens as opposed to creating offense for himself (which is why he's a good 2nd option, not first).
The Paul Pierce comparison is legit. They both aren't volume 3 point shooters (not in the beginning of Paul Pierce's career), but they shoot a very good percentage. He is not extremely athletic but has good lateral quickness and just has a feel for the game. DWill 2.0 is very crafty and he can pretty much get to the hoop any time he wants. And here's something that translates into the NBA - he gets to the foul line the way Pierce does, meaning even when he is having an off night (which he pretty much never did in college), he can still find a way to score - and you need that in your go-to scorer.
In the beginning of this past season, when he was barely a lottery pick on most mocks, John Hollinger wrote an article on how ridiculous DWill 2.0's PER is from his freshman campaign, and how difficult it is going to maintain that. Well guess what, he did, and improved on his outside shooting - his previous worst attribute. He is also a clutch player who made many many MANY big plays down the stretch in every game... not just in the NCAA tourney.
People are concerned about him being a tweener, but I'm not. There is a reason why despite being a 'tweener', he is still a possible #1 pick. Someone mentioned earlier that he hasn't seen anyone who improved that much over his two years in college, and I agree wholeheartedly. I'd compare his development to Russell Westbrook (from my alma mater UCLA), they were both 3 star recruits out of high school, and improved so much in first two years to the extent that they kind of "have" to go in draft a year early. And people were also concerned about Russell Westbrook being a tweener, and look - he is now one of the brightest young star in the NBA, and I fully expect DWill to be the same.