If anyone is worried about missing Alex Brightman, I've seen the show twice, once with Alex as Dewey and Mamie Paris as Rosalie and once with Will Blum and Sierra. The show is still a lot of fun with Will. His interpretation is different but also very entertaining. Alex plays Dewey more high intensity and manic with more abrupt emotional switches and sharper emotional peaks and valleys than Will. Obviously humor is subjective, but I found that interpretation to be a bit funnier and thought Alex's comedic timing was slightly better. That's not to say Will was not also quite funny though (he is!). I strongly disagree that Alex is doing a Jack Black impression. There will always be some similarities on aspects that are most intrinsic to Dewey because they are playing the same essential character, but both Alex and Will totally make Dewey their own in my opinion.
Will's voice is great tonally but was less powerful. To be fair, I think I saw Will in the second show of a two show day for him and I saw Alex on a one show day. My friend and I both agreed the romantic chemistry was better between Will and Sierra than it was with Alex and Mamie. I had a much easier time buying into that subplot, which I think is a touch underdeveloped, with Will. Will is very talented, capable, and excellent as Dewey. My friend liked him and Alex equally well, and both left me grinning and excited after the show.
For me, however, Alex just is really spectacular and an awe inspiring ball of energy. Everything popped a little more and felt more vibrant with him on stage, and I felt like he raised the level of the entire show up a notch by bringing out the absolute best in the rest of the cast as well. He is so intensely emotionally present at every moment that one of my favorite scenes when I saw it with him was actually the bedroom scene with the kids at the end when Dewey doesn't even say or sing anything for a while. It's a scene that easily could play as cheesy and overly sentimental if handled wrong. He goes through such an clear emotional journey in the background and has such strong, heartfelt, honest feeling connections to all the kids in that moment that it actually made me tear up a bit. To sum everything up, I would recommend seeing Alex if you have a choice, but I wouldn't hesitate to highly recommend seeing it with Will either.