If Bethmann-Hollweg's plan had been a good one, WWI wouldn't have spread, so I guess I'll have to say it was not a good plan.
On the one hand, the plan was politically good because it did what Germany and its allies needed. To keep German opinion and German allies happy, Bethmann-Hollweg needed to look like he was standing up to the Slavs and supporting Austria-Hungary.
But he did not account for the opinion of other countries, specifically Russia and England. Bethmann-Hollweg thought that Russia wouldn't get into the war and even if it did, the British would not come in. He was wrong on both counts.
So I think his plan was politically "good" but it was a disaster because it helped spread the war.