Upgrading from a HDD to a 120 GB SSD is worth it. However, the 250 GB and larger SSDs tend to be faster than the 120 GB models, so paying the extra $10-$30 for the larger SSD is usually worth it.
SSDs are basically like putting a bunch of USB flash drives together. A single flash drive might only be able to write at 70 MB/s. But if you put 8 of them together, split the file into 8 pieces, and send one piece to each flash drive, and each drive writes their piece simultaneously, then your overall write speed is 560 MB/s. The 120 GB SSDs typically use half the number of internal flash drives as the 240 GB and larger SSDs, so will be a bit slower.