Depends when it is. 90% of the troops died on the march, but it was useful, because the Nationalists didn't know where they were, and could conduct underground operations. If they are attacked at the start, there will be high casualties on both sides, and while destroying the brunt force of communist forces, troops that escape could share the word of Mao. If it happens near the end of the march, the remaining exhausted, and the small number of forces, will all die fairly easily.