No, bad idea. Regular hard drives are already excellent for storing and transferring larger files like movies. They are huge and can transfer ~100 MB/s sequentially.
SSDs are only really good for randomly accessing small files, e.g., your operating system and applications. If you want to upgrade an iMac, I would put an SSD in an external enclosure, connect it via FireWire or USB 3, and then use that as your boot drive and use the iMac's internal 3.5" drive to store larger files.
I couldn't disagree more - hard drives are slower in SSDs in all instances. Sure hard disks sequential read is ok but a SSDs will be 300MB-400MB/sec. Small files are better due to high random IO performance, no fragmentation to deal with on SSD, etc. I'd happily ditch hard drives as quickly as possible.
256GB SSDs are already approaching $150 on the internet, they're really not that expensive anymore.