I've seen one magus in play so far in my games.
The advantages: It hits hard (when using spellstrike), it gets good buffs, and the self enchanting is awesome. In fact, it was keeping pace, damage wise, with the (moderately optimized) Barbarian overall. But that was with using valuable per-day abilities, when the barbarian was actually not using her Rage most of the time.
Disadvantages: The magus found it much harder to hit opponents, overall. Plus, they are very dependent on multiple attributes.
Overall: I think it's nifty, but hardly unbalanced. Decent hybrid class.
Now, does it replace the Eldritch Knight? I don't think so. The EK can be built, at 20th, to have a Caster Level of 18 and BAB of +15, just like the Magus, but still have 9th level spells. Sure, they don't have the ability to cast in armor, but spells like Displacement help, and you can make do with Bracers of Armor and similar items. Or you can go the other direction, and focus on the melee aspects, where you can hit a BAB of +17 and several fighter bonus feats, if you so chose. I'm assuming you take max levels in EK, to make this clear.
On the other hand, I almost see EK as a better bard prestige class than for a fighter/wizard. I don't see the Magus as better than the EK. It's just different.