A dog as King Lear is probably the only thing that could make me enjoy that play.
As for McKellen as Hamlet - as far as I'm concerned, there are so many freaking productions of Hamlet done all the time, with Hamlets of every race, age, and gender, that pretty much anything goes. If it's a dumb idea for the text, then at least audience members will get to see McKellen do his thing for 3 hours. And if that's not enough reason to see it, then you can always wait a few months for the next production of Hamlet to come along.