While X spells typically count X as 0 in their Converted Mana Cost, as soon as an X spell is placed on the stack, whatever X is made is added to its Converted Mana Cost. This means that if you set an X spell such as Bonfire of the Damned (Casting cost of XXR) to 5, due to it having two X’s in its cost, that Bonfire is treated as having a CMC of 11. What this mean is that by having lots of X spells in the deck, you can scale up on experience counters rather quickly. Also, even with the casting cost reduction, the Converted Mana Cost itself doesn’t change, as it only affects how much mana you actually pay. Therefore, you could play Blue Sun’s Zenith (Casting cost XUUU) to 5, with 7 experience counters, cast it for 3 Blue mana, and still get an experience counter due to the CMC being 8.
As a compliment to an Izzet (Blue/Red) Commander such as Melek, Izzet Paragon, Mizzix could be quite a deadly force. But on her own, Mizzix has plenty of strong X spells to draw from. The most obvious play alongside Mizzix would be the powerful Epic Experiment, which could essentially be cast for only UR, and cast X spells without having to pay their mana costs. But there are many others. The best of these include:
Banefire, Blue Sun’s Zenith, Bonfire of the Damned, Braingeyser, Clash of WIlls, Comet Storm, Commune with Lava, Condescend, Curse of the Swine, Devil’s Play, DIsplacement Wave, Distorting Wake, Goblin Offensive, Invoke the Firemind, Mindswipe, Red Sun’s Zenith, Repeal, Rolling Earthquake, Rolling Thunder, Stroke of Genius, Tempt with Vengeance, plus many more!
Another card that has been suggested is Call of the Skybreaker, which has a casting cost of 5 colorless mana and two Blue/Red hybrid mana (7 CMC). It has Retrace, meaning you can cast it from your graveyard by discarding a land card in addition to paying its mana cost. But with 5 experience counters, you get a 5/5 flying token for (U/R) (U/R) and discarding a land you probably don’t even need. This is an awesome way to get some big threats on board. Of course, there’s also Talrand, Sky Summoner, who can create a whole ton of 2/2 flying Drakes in such an instant and sorcery heavy deck.
Mizzix should be a good friend of instant and sorcery spellslingers as a member of the 99. But if you are to build a deck loaded with X spells, Mizzix could be very interesting. You don’t need all that many experience counters for this deck to do plenty of damage quickly, and having Storm spells such as Grapeshot and Empty the Warrens can end the game if you cast enough spells in one turn. While Melek, Izzet Paragon will probably remain more popular, Mizzix has a lot of potential.