depends where you use it - some are aluminum directly touching your hard drive casing - great heat dissipations. others have no fans and totally enclose it - you'll burn it out pretty fast if it's not properly standing.
I use these, too. There's one in my toolbox for whenever I upgrade my laptop. I use one to siphon the old data off, then wipe the drive before putting it back into the laptop and selling it.
They're not too fast; ATA-66 at best. I don't think it's good to use one permanently, but they're great for cleaning up and fixing.
Yeah i plan to do what u do Mike... but in time i would like to findout if anyone has used one premanetly like for a HTPC or small computer... thanks again guys i plan to purchase one soon for my laptop hds i have from family memebers...