I'm a DM. I improvise more than I follow adventures. I make on-the-fly rulings all the time. I try to be fair, be on the players "side," while allowing risk of failure. I make clear rules at the beginning of a campaign on what classes, races and other options are available, and I'm not afraid to straight up tell a player that his character idea isn't going to work in my campaign, even if it seems to follow those rules. I like wild mages.
And I am not satisfied with the lack of direction regarding "...DM can..." in the Wild Mage.
It essentially means a player has no idea coming into a campaign how his class features are going to work. Before someone brings up how that is "the point" of wild mage, let me say that I think there is world of difference between the character not knowing what's going to happen, and the player understanding how his class features work. A class feature that essentially says, "this works however the DM wants it to work" is barely a class feature at all. It certainly isn't something one can rely on understanding when creating a character.
So say players and DM talk it over when making a character. Regardless of how obvious and clear it may seem at that point, and how much they may come to an agreement, the purely arbitrary nature of the DM administering an important class feature will cause a lot of DM-player friction and player dissatisfaction in many campaigns. It's like friction over paladin requirements, only faster acting.
It should say at the beginning of the Wild Magic section: "Wild Magic is unpredictable not only for the character, but for the player also. The chaotic forces of wild magic mean many players will abandon the class before gaining their 3rd level. When you play this class, your DM can, each session, really piss you off a variable number of times..."
I'm just not buying if it what we have is all it says. If they at least have a sidebar telling the DM how he should handle these wild surges (even if that means giving examples of different ways of handling it and different frequencies) it might be okay. Even then, it's poor form for a class feature.