After reading Mark and McCrankpin, I started to think (doesn't happen often, so I let it run... ). I've had mainly bad, but also a couple of good bungees. This is what I (think I) have recognized in high-quality bungees:

1) MAIN feature: they are not so elastic in the sense that after a certain point, you definitely cannot pull any more because the outter fabric covering the rubber stops any advance. At that point, they feel like a regular rope. I guess it's the way to protect their (elasticity) properties against over-stretching (it comes to my mind when you pull too much spring and it never comes back agian). Cheap ones feel you can still pull a little more.
2) That outer fabric feels very solid, could hold many kilos under extreme tension (mine are black, fabric and rubber, btw, was Mark said! cheap ones' rubber is usually white)
3) They are thicker (the rubber).
4) Hooks are much thicker and solid, you feel no way you can bend them with your thumbs.
5) The endings are better cut, fixed and fit better in the hooks.
6) As said, you get them in your hand and you already feel they are way better.
7) They are more expensive and don't come 10 in a bag with different lengths.

Does it fit with your experience? Please correct me if I'm wrong (I haven't tried/found many good bungees).

Why I do feel that ROK straps are better:

1) VERY solid.
2) You don't have to mess looking for a different place to hook/pass the bungee through when the luggage/parcel(or scuba bottle! Oh, man, that's faith not only in your bungees!!!) is bigger or smaller (or has more O2 liters!) to have a good fit.
3) They don't scratch anything (hook caps tend to fall, is it only me or cheap ones?)
4) You really decide the tension (linked to point 2), since you pull the strap going through the buckle. And you can pull a lot, so everything is rock solid, well attached.
5) The flat surface shares the tension over a wider area, so they do not embed them into your luggage as sometimes bungees do.
6) There are no hooks (I add to the urban legend, but a friend assures a knows a guy who got one-eyed do to a bungie; after seeing his scary face when he saw me packing lousily and shouting "WATCH OUT, always out of the return path!", since then I really take more care/believe him/got Rok straps). If you release the strap by mistake, since the rubber part is quite short, they won't fly back (if you save your eye, you can still angle your ear!)
7) They don't pay me (yet), but after this speech I'll ask for my commission!

SUMMARY: A combination of a nylon strap with a bungee, with the good things of each.

Disadvantages:
1) In very rare cases you may struggle to find a place to loop them. It happened to me on a Honda PCX125, where it was almost impossible to fit anything (no hooks, nothing protrudring, only a slight lip around the seat), but bungee worked (bad, but better).
2) You need faith in the buckle. If it brakes or releases, your load will definitely fall (but you still use them in pairs). So I would not leave them parked under the sun innecesarily to protect the plastic. Good bungees seem to resist anything, as said.
3) I feel the outer fabric covering the rubber may be a little less sturdy.
4) Not very cheap...

Esteban

PS: I feel overanalysing, like writing "what should be the attributes for the perfect thimble?". Man, it's only a metal piece on the tip of your finger!!!!