I've used a thermaltake USB3 enclosure on my SSD and it still gets full speeds just cheaper than the thunderbolt one.
2011 iMac, so no USB3
I am interested in more info as this is the approach I am going to initially take.
Dead easy. TB cable, Thunderbolt sled, SSD. I believe anything above 256GB _may_ cause problems due to additional power requirements, resulting in the drive disconnecting and various other niggles.... MacRumors forums have a bit more detailed information.
Other than that, I just booted from an ML USB stick, fired up Disk Utility and blanked the mechanical drive then installed ML to the TB-connected SSD.
Once you're installed, use Trim Enabler to switch on TRIM if your SSD requires (and after every point release of the OS, as that detects a non-factory SSD and disables it).
Although it's possible to use CCC to restore a full backup to the SSD, I didn't bother and preferred clean installation.
You get a short delay on cold boot while it works out where the bootable OS drive is connected, but other than that it's pretty much full speed. Wake from sleep mode
I didn't bother relocating my root user data stores and simply shifted my iTunes library and iPhoto library to the mechanical when restoring those.
That's pretty much it really. It just works after the initial setup!