If they do this right, it's going to be fantastic. Purple Man is an ass - complete mind control with no restraint or morals, so he could be completely evil and depraved (as he is in the comics), and that would be an awesome character for DT to play, a more intelligent and cruel version of Feldwebel Kurtz, hopefully. I've been hoping to see him play something more action-y for a while (I've loved everything he's done for the last few years, but I've missed that bit of the fantastic that we haven't seen since 2010), so this is welcome. And while he does evil and insane so well, his mainstream fans haven't been exposed to him doing it very much, so that should be a wonderful shock.
I have to admit that I am surprised that he's landed a role in the Marvel Cinematic Universe. I really didn't expect it. He's been doing serious drama and light comedy for so long (ever since he left Doctor Who) that it looked like he wasn't interested in doing sci-fi or fantasy anymore. And I suspect it's really difficult to get into the MCU, because it's so big and popular right now.
EEEEEEE! He's going to play a villain! That is just so exciting!