It would be acceptable with commas:

I, personally, ...

Though, it is really a spoken form, resulting more often than not from false starts. If one were to write this, they should select:

Personally, I ...

Even then, there is some question about its necessity. It is reflexive and in most cases tautological. It would probably serve to add nothing more than emphasis to a sentence.

I think you should ask a doctor. Personally, I think you should ask a doctor.

There is really no semantic difference between these two.

It would be acceptable with commas:

I, personally, ...

Though, it is really a spoken form, resulting more often than not from false starts. If one were to write this, they should select:

Personally, I ...

Even then, there is some question about its necessity. It is reflexive and in most cases tautological. It would probably serve to add nothing more than emphasis to a sentence.

I think you should ask a doctor.
Personally, I think you should ask a doctor.

There is really no semantic difference between these two.