You'll learn about many core mods of FTB, but considering the current pack, that's not the optimal way to learn. The FTB 1.2.5 map depends a lot on EE2, which is not in the new pack nor will it ever be, seeing as it won't be updated, being replaced by EE3, which for what I know is really different and more balanced. Also, the new pack contains a lot of mods that have been released/gained popularity recently, and therefore are not present in the old pack, such as Thermal Expansion, Factorization, Gregtech among many others.
That being said, I still recommend it, it's a very fun map and it does have a lot of mods in common with the new pack that are essential for understanding the synergy between all mods (IndustrialCraft, Forestry, Buildcraft, Redpower).