Here are my 4 stages of Magic-User Characters for most generic level-based fantasy pre-4e RPGs:
One-Trick Pony. 1st level or thereabouts. Let’s face it: it’s one Magic Missle or Sound Burst spell and then it’s poke, poke, poke with your staff or dagger. At this stage, you are most akin to one of those “fire n’ forget” rocket launchers certain militaries employ. That Stupid Fighter has to keep you alive most of the time, and boy if he doesn't remind you about it.
Utility Infielder. Generally lower levels. While you don’t disintegrate your enemies by the bucketful at this stage, you are an amazingly useful little creature. You Grease to cover a getaway, Purify Water when you’re adrift in the middle of the ocean, and you’ve probably learned at least one little illusion spell right now. You also can summon things like Dire Weasels and Celestial Badgers, which is a lot more useful than it sounds. That Stupid Fighter occasionally even thanks you for helping out—imagine that!
Self-Sufficient: Intermediate levels. At this point, you can easily take on a small mercenary squad, a couple of monsters, or a little dungeon crawling without expressly needing That Stupid Fighter to cover your rump. You can probably conjure any supplies you need, and you’re a good enough fighter to make average men-at-arms think twice. Those d4 Hit Points do add up, eventually.
The God of War & Death: Every game seems to have a level where this becomes a reality. That Stupid Fighter kills, what, 3, 4 warriors at a chop at best? You can kill 3-4 armies, easy. You rain meteor swarms, heightened elemental atrocities, and other spells upon your enemies, many of which have only the bleakest save vs. Instant Gooification. You have probably cleaned out at least one of the Nine Hells. The only enemy you still fear? Metal armor.