[3.5] What do you think of Mystic Theurge?

[PhoenixRivers]

It really doesn't. You're still quite powerful, but Wizard 20 is much more so.

Let's look at this:

Raw power at level 20.

Wizard 3 / Archivist 1 / Mystic Theurge 10 / Wizard + 6

Now, you can be either:
Wizard 20

or

Wizard 19 / Archivist 11

The latter is better.

without EE:

Wizard 17 / Archivist 13

That is better than most wizard 20 builds, just due to the added versatility combos.