Was in Taiwan for 6mths just after the 921 earthquake and visited Alishan during the period. Felt that it was a big letdown. Perhaps it was becos i went during the off season and it was foggy and rainy.
However, feedback from some of my friends were also poor. Basicially, I dun find the place exceptionally beautiful to justify the trip there. Think it is a overrated place (perhaps from the Alisha de gu niang song we all know).
In any case, there is a gallery posted recently in the Travel and Landscape on Taiwan and the Alishan pictures are very good. You might want to ask the poster for his/her opinion.