2.5" IDE drives get their power (+5V) from their main IDE connector.
Amazingly , 2.5" IDE to USB adapters know this and supply the power the host's USB port to the drive, or from a power unit that plugs into the USB adapter.
Transfer speed will depend on your interface, with eSata being the fastest, then Firewire, then USB. Some of these docks are USB only.
I use a Thermaltake dual dock with USB/eSATA interfaces and it is great for imaging or cloning. One thing to note however, that hasn't been mentioned yet, is that these docks are only for 2.5" and 3.5" SATA drives. I've not seen one yet for the mini and micro SATA drives.