Seeing as the new single rubber attachment piece is becoming more and more common in the Lego sets (42069 set has it, for instance), I've decided to test how much it improves the grip as compared to bare tracks. Gotta say, the results are impressive:

 

Nice test, as always!

I recently bought a whole bunch of these attachments to test them on my 42065. Mostly because most of (scarce anyway) area in my apartment is either covered in tiles or wooden flooring and the tracks were sliding crazy.

I bought one for each link, being convinced using two would cramp the turning performance. I kept the two "stock" M motors. I stuck them in a row on each track because of my obsessive personality traits combined with the fact the odd number of links in each track. I used the internal row first but then the attachments "drew" longer walls of an rectangle. The turn performance was much weaker than with no attachments. So I decided to try an external row on each track - then, the rubber attachments "drew" two walls of a near-square. Performance was way better. Of course, bare tracks have better skidding performance and the perform better on blankets, towels and other "fuzzy" materials. But on the hard flooring the acceleration, speed and handling is way better. Also, the climbing (especially over the obstacles like a vacuum pipe etc.) performance increased. Then I tested them set in an alternating fashion and the performance was cramped and it caused too much vibrations.

Anyway, I play around with the modified 42065 around my house only, so I consider those rubber attachments a great investment and I gladly traded the cons for pros. To sum things up:
CONS:

• skid speed quite reduced on any surface
• turning speed a bit reduced on any surface (except super-smooth ones)
• performance on "fuzzy" surfaces deteriorated greatly

PROS:

• almost no sliding on wooden and tiled flooring = acceleration bettered
• handling bettered
• climbing ability improved
• straight line speed increased
• track links deterioration rate greatly reduced

If you're curious as to whether the rubber attachments themselves are of good quality or tend to pop out - YES to first and NO to second.
Haven't lost a single one yet, they sit firmly enough to not get torn out by torque or climbing, yet they are quite easy to remove by hand. Haven't noticed any wear on them yet either.

Well, it is clear they are good for indoors, but what about outdoor driving? 

I've been wondering for while if these were worth buying to improve the perfomance/playabilty  of my 8275 and 8043 on wooden floors. After watching this I'll be putting a BL order in shortly.

Thank you for taking the time to do this comparison.

Great test, this has definitely increased my wanting of those rubber attachments .  

I never knew those red double-attachments were so rare. I got some from my EV3 Expansion Set.

been using them since they came out....fantastic review Sariel!!

Thanks for the test Sariel, really useful! I will definitely keep an eye on those small rubber attachments, and now I know, that 1/link is enough.