What happens to the Bearer if the weapon is destroyed? I'd assume they just become an ex-Bearer, but I think you should elaborate on that a bit. I'd also guess that Soul Merged Bearers would take some other kind of penalty if thier weapon was destroyed somehow, and I'd also think that they would get some sort of bonus against sunder checks. You also forgot thier BAB, though considering how combat-oriented this class is I would guess you had full progression in mind...

Also, I'd say overpowered... A +5 Keen, Adamantine, Flaming (or other energy), Brilliant Energy, Vorpal, Epic, Symbiotic, Intelligent weapon at level 10 Bearer? I'm not even including the additional bonuses that go to the user there, such as the plus to hit and the damage reduction... Seems a bit much to me.

I'd suggest you take a look at the Shining Blade of Heironious PrC in Complete Divine for ideas on balancing this; that class has a similar feel to it. I'd say that particular class is maybe a bit underpowered myself, but it's got some ideas that I think this class could incorperate.

Balance aside, I really like the flavor behind this class. An intelligent weapon and its user becoming so closely bonded that they actually become one entity... It's a good concept. When I start DMing, I might make some villains from this class...