I'm relatively new and have trained one mage to 108.2 (and climbing, would go faster except I'm pursuing Honor and there's no mage weapon fun safe there
) and have another char just past 90 by using the spellcasting mage weapons strategically based upon their negative rating and where my skill level vs. success at casting falls -- they esp came in handy during the mid 70s to GM hump where trying to cast my true-level circle spell could mean fizzle and death. For example, the difference between a -25 and -29 seems silly until you realize the % of successful casts of of 6th circle explosions or energy bolts (good bang for your mana dollars) at magery 72 (50%), 82 (75%) and 92 (100%).
The best way is to equip one according to where your actual level is, cast reflective armor or otherwise get your phys resist to 70 and head to Despise with its lizardmen and ettins/earth eles with some good DCI/HCI/PhysReflect jewelry or related mod armor and shield in place. (I recommend these non-magic casters so you can defend against one kind of damage.)
I also recommend the relatively low cost "arty" called the Staff of Power, which is a -15 and the Jaana's Staff at -20 (which you'll likely earn if you find spawns to play with in anti-virtue dungeons),as well as the -10 weap that you get completing the new player magery quest in New Haven to help find those magery sweet spots and complement the random looted mage weaps.
Finally, the actual magery gains I receive with a -29 lizardman slayer mage weap (w/o spellcasting) were negligible, by the way, but it was fun playing melee as a break to EVs. (I have high anatomy so I actually can do some damage w/weaps even w/o tactics.) Last recommendation: close the skill menu for a couple of days and just play somewhere -- I thought I'd hit a gaining wall about 102 until I stopped paying attention.