I unlocked my Wizard a few days ago and have advanced him to Specialist level so I don't have access to all of his upgrades just yet.
My impression is that the Wizard is primarily designed to shine in Co-Star mode- it's easily accessible for the most part and only requires a bit of adjustment whereas I'm still getting used to him in his default state, mainly his primary weapon.
In his default form I find his primary weapon requires a high level of precision and I'm still trying to decide if his charged shot is worth the extra time and effort, but there may be an upgrade I haven't tried, or haven't unlocked, which could influence that decision.
I'm still working on figuring out the timing and aim of Zee-lixer but have used it successfully on a number of sprinting, bunny hopping Peashooters and to protect against Chili Bean Bombs and Super Sap Traps.
As far as Co-Starring on Boss characters, I don't think it is as problematic as some have presented it. I've both Co-Starred on Boss characters and gone up against such as a Plant and from either perspective I don't think that a Wizard doing so radically changes the outcome of the match because whichever team wound up having the higher level of skill, coordination or teamwork continued to be the winner regardless of whether or not a Boss character in the game was Co-Starred by a Wizard.
I have noticed that more tactically aware players have started specifically sniping at the Co-Star which can make it difficult for the Wizard to provide the kind of support for which he was designed. I've been sniped off of someones head on several occasions and have done so myself as a Plant.
I think my opinion on this is much the same as other issues where certain complaints are leveled at a certain character, and I'm not dismissing the fact that there can be legitimate balancing issues, but generally I think if a particular Hero class if giving you large amounts of difficulty then you probably aren't employing the right tactic or the right character to counter that Hero's abilities, or that particular players playing style. There is also the simple fact that some people just get very, very good with one Hero class or one play style and it's more that players experience and expertise which unbalance a match and less the character they are playing.
As far as introducing characters to both factions simultaneously, that would be ideal but the unfortunate fact is that since the characters are clearly going to be introduced through the Prize Map in order for two characters to be dropped in simultaneously as "end prizes" they'd probably need to arrange the Map to drop out one or two of the higher cost cosmetics and possibly change the map route rather than rely on the default template. I'd wager if they dropped both at once we'd also wind up with players having to complete either most of, or all of, the Map in order to get both and there's already enough people complaining about the total number of Bulbs needed to get everything before the end of the month as-is without the threat of losing one new character to Rux's inventory to inflame things.
In GW2 Torchwood and Hovergoat were simultaneous prizes because you have to invest a ton of time in waiting on random drops and then honing certain skills to even reach the room where they were imprisoned and that was just a very different way of including the characters in game play.