In summary, Zwei is a character heavily dependent on the player's reads and predictions. His tools do not allow him to be as much of a "button mashing" character as others.
His main strenghts are GB damage, set ups, and overall damage; however, they each come with a trade off. For instance, all his GB moves, with the exception of 66A+B, have a loophole which the SCV roster can exploit. This applies to his set ups as well. His damage, though it can be extended, gets affected by the scaling, reducing it to a large margin.
To list the main weaknesses, speed, stepping, and range would come in first. Nevertheless, by making good reads, as mentioned above, these weaknesses can be compensated for.
Zwei is not a low-tier character, but by no means is he top-tier either. He is found to be somewhere in between the middle. It all comes down to the how/when his tools are applied. A zoning playstyle will not be as effective on a slow character with massive range than on a fast character with little reach.
Concluding this, like all other characters, Zwei has a great potential, and as such he has the tools to prove that so. The only difference is that his tools are much harder to apply, and therefore they cannot be tossed unconsciously, but they are of great effect... when used correctly.
-Zane