JeniSkunk: Interesting that you mention a Pixie familiar. I chose that as well.
That's actually the least useful familiar choice for you.
Pixies can pick locks and disarm traps. Which is very, very useful if you're not a rogue. But since you can already do that yourself...
Then again, most other familiars won't be much good to you either, as they'll be level 2 and you're level 10.
Melf's Acid Arrow, and Knock are ones I'm looking forwards to adding for the next Wizard level-up (Wiz 3). As for Find Traps, both the spell (Wiz 4) and scrolls are not so useful, since I can already do that.
Right now, she can't dish out the spell damage in big numbers, but with my current playthrough still being in the first part of the OC, that's not as big an issue as it would otherwise be.
The issue is that you'll
never be able to deal spell damage in big numbers. You'll
never be able to cast disabling spells that the enemy won't save against. By the time you do get Acid Arrow, it will deal so little damage most enemies won't even notice. (10.5 average) Your own sneak attacks are already twice as strong as the Acid Arrow spell.
In comparison: A real (single-class) wizard at level 11 would be able to cast Isaac's Greater Missile Storm, which against a single target would deal an average of 77 damage.
It's not a complete loss: you can cast some quite useful defensive spells that last a few minutes real-time. Endure Elements probably still helps at this stage, Protection from Evil is also useful. Even Shield could come in handy.The problem is, though, that you could get the same benefits by buying a few (dead cheap) potions or wands. Your wizard level is so low that potions are actually better than what you can cast yourself.
I'm sure that you can still survive. Most enemies you face are rather weak, and if you're clever in using traps and such you can probably beat tougher enemies. But I expect you'll really struggle later on, either against the swarms of high-hitpoint monsters you'll face, or against bosses and enemies that are immune to sneak attacks.
Perhaps you won't mind the really long, drawn out fights against random enemies on the road you'll meet later. But I remember that I got very frustrated with my poorly-designed multi-class wizard when I played this game. (Admittedly not helped by the fact that the original campaign isn't all that good.)
You may also run up against fights that are essentially unbeatable with your character, unless you do the "set 100 traps first" thing. But as Jonann describes, that gets rather boring.
While I could easily respec her, to rebalance Rogue/Wizard, or completely remove the Wizard stuff, I had planned the build to be 4 Rogue then 1 Wizard, planning on topping out at Rogue 32/Wizard 8.
You won't reach those levels, actually. Not in the OC.
Epic levels were added in the Hordes of the Underdark expansion, which is meant to be played with a character that has finished the Shadows of Undrentide expansion. But you're supposed to start SoU with a fresh level 1 character, and not even HotU will take you up to level 40.
The OC will bring you up to about level 16-18. It depends on how many side-quests you do and whether or not you bring henchmen/summons along.
SoU ranges from level 1 to level 12-13. HotU starts at level 13-15 and goes up to level 25-27, again depending on how much stuff you do/companions you have.
However, there are some user-made campaigns that reach all the way to level 40, including some that you're meant to play with your HotU character, and some that start at level 1. Well, one. Aielund. There may be more but I can't recall.
Online persistent multiplayer worlds often go up to level 40, but in almost all cases you can't play them with your offline characters. You have to make new ones specifically for those servers.