When one thinks of a magic-user in fantasy literature, or its sources, one often thinks of the figure that itself stems from various strands of Western systems of high ceremonial magic and the mythology surrounding them. Of those sources, one pattern that emerges that that the successful and competent practitioners are those who comprehend the reality of how magic works--its fundamental principles, confirmed by repeated experimentation over time and in various places--and then seek to apply that knowledge to create wonders heretofore impossible to achieve in any other way. In short, they are scientists and engineers and the facts that they discover become the basis for new and improved technologies--new spells, rituals, ceremonies, etc.--that others can employ and benefit from. In short, these magic-users are people who--for all intents and purposes--approach magic as just another science.
In the core rulebook we have the Ley-Line Walker, the Shifter, and the Techno-Wizard. Each of these Occupations are expressions of the aforementioned archetype, with different elements emphasized. The Walker focuses on casting spells, the Shifter on ritual magic, and the Techno-Wizard on magic item creation. Yet each one is capable, to some diminished capacity, on doing the others' thing; this is not the mono-maniacal focus that one sees in craft traditions. These Occupations other notable abilities are direct derivatives of their core capacities, sample applications of the principles that are fundamental to that Occupation's discipline as well as indicators of the basis for their body of knowledge.
Magic-users of this sort find a lot in common with Body-Fixers, Cyber-Docs, Rogue Scientists, Rogue Scholars, Operators, and similar mundane scientific and technical Occupations that deal in non-magical science and engineering pursuits as well as professions (such as law) which promote a similar mode of thinking and reasoning in its practitioners. Therefore, playing such a magic-user should involve the player getting familiar with how such people think and behave in their work lives; there is a commonality of thought and behavior for these professionals, even if they exhibit a wide array of behavior in other areas of their lives, so knowing how they think and do will serve you well in maximizing your gameplay experience.
The other thing that magic-users of this sort often do is pursue research. While pure research is good for setting verisimilitude, and Game Masters are wise to pay attention to this both for adventure potential as well as for campaign management, players are advised to focus upon applied research; they should focus on application of discovered principles instead of seeking out new principles, which translates into creating new magic technologies (spells, rituals, items, etc.) and techniques for their use. Creating new technologies is one of the best ways for a player to distinguish his magic-user from all others, so it would be wise to learn what rules exist (or make new ones if not satisfactory) to do just that; remember that these creations are intended to be verified by recreating the process, so having a sound system for doing so is a feature and not a bug.
Your magic-user will need downtime to get the full benefit of his skills and knowledge, and he will need a place to do research and conduct experiments; Game Masters should be friendly to these requirements and accede to players' wanting to establish such things for their magic-users. In particular, the Game Master should work with players to establish what the magic-user needs to do to accomplish the research objectives. As has been the case since 1974, these sorts of magic-users are campaign-drivers in their own right when both players and Game Masters exploit this potential. The other Occupations in the game, and their variants, of this sort show the incredible diversity available for those willing and able to make use of it; I recommend taking the time at and away from the table to make this happen.
Starting next week, I'll move into more specific examples.