My OWC Mercury On-The-Go Pro Portable enclosure was as pretty as all get-out, and it moved my test data at 575Mbps via FW800, but...

In this old video, you can see (at a bit past 2 minutes) how snugly the circuit board fits into the heatsink, while in this new pic you can see that it "fits" (and I use the term awfully loosely)...well, awfully loosely. (It's more or less stable in the fully assembled enclosure, but it's ersatz stability; it rattles around if you shake the unit.)

Per OWC tech support, "...the video was...using an older circuit board which is a bit thicker than the current board we are using. Our current board is within our specifications though."

It appears, then, that OWC changed the board's specs at some point without correspondingly re-engineering the heatsink (presumably to save money).

As far as I'm concerned, well-engineered devices should never have loose-fitting components!

Is that unreasonable?

PS: It's to OWC's credit that when a replacement enclosure confirmed that my problem was with a designed-in flaw, and "[it's] within our specifications" didn't satisfy me, they unilaterally upgraded me at no further cost, and after 6 1/2 months.

IMHO parts rattling around in a mobile enclosure are undesirable. However depending on the device very well engineered devices may well have loose fitting components. In fact that may be an intentional part of the design. A classic example is the M16 rifle. It rattles like crazy and is designed that way so that dirt, sand, mud, etc. that will get into the mechanism do not jam the weapon.

Just a side note on heat sinks in external enclosures. For many years heat was considered the mortal enemy of hard drives, but Google Labs extensive study of hard drive failure concluded there was no provable link between heat and drive failure. I just got a neat little external enclosure from OWC that is entirely made of plastic and has no heat sink at all. It does have some tiny ventilation holes but not enough for any serious air circulation. I haven't had it long enough to draw any conclusions about drive life, but it works well and costs less that $20. But it does not rattle.

If you're referring to their On-The-Go enclosure, I've had one since September 2012 with a Seagate 320 GB drive in it, and can confirm that heat is not an issue. Not even close.

And, of course, there is the bonus of it being so funky looking.

Thanks for the insight into M16s; it almost makes me want to enlist so I can tear one down and see what's what. (Really; I'm that curious!)

My replacement OWC Mercury Elite Pro uses an "heat dissipating (aluminum) enclosure" rather than a heatsink, and it wasn't terribly hot after a 2 hour surface scan (which would be an interesting test of your new enclosure).