Whoomp! ships in two sizes, with two pairs per package. You can order them in a package with four standard, four slim, or two each standard/slim. A very nice black fabric/leatherette zippered case is included, along with ear placement and fitting instructions.

Comfort is personal, and Whoomp! feel fine for half an hour per session in my ears. Bass response is very good, but you'll have to tweak your iPod/iTunes equalizer settings to add missing treble, as desired. Be patient when affixing Whoomp! to buds, because it may take you a couple of attempts to place them precisely. Ditto when removing Whoomp! so you don't accidentally pull off the top grille of the Apple earbuds.

To evaluate Whoomp! I broke down their criteria, as follows:

PROS
Excellent case
Rich bass
Reasonable price
Acceptable noise cancellation
Secure in-ear fit

CONS
Possible discomfort
Possible incorrect placement onto Apple earbuds
Possible tricky insertion for correct angle and fit
Need to purchase additional Whoomp! once or twice annually

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Regarding usage and replacement, Dr. Robert Oliveira Ph.D., Hearing Components' President and CEO, tells MyMac.com:

We have found that, unlike the typical 3 months of use with the Comply Canal Tips for the Comply NR-10 model (you previously reviewed), the Comply Whoomp! earpieces typically last for more than 6 months of use.  The Comply Whoomp! earpiece is designed to comfortably cradle in the bowl of the ear with the foam sitting only at the entrance of the ear canal...not deep in the ear like the Comply NR-10 foam pieces.
 
This accounts for product use life being more than 6 months not the 3 months typical for the Comply Canal Tip and it also results in the lower noise reduction. This is an important Comply Whoomp! design feature since we feel it is desirable in many situations to improve comfort for the earbud, to have some noise reduction to allow lower volumes to be used, but to not isolate you from your environment. Our Comply NR-10 earphones with their much higher noise reduction is designed for those wanting more noise isolation; like flying on a plane, exercising in a gym, or studying in a noisy area.

Since many users of Apple iPod Earbuds force the buds into their ear canals in an effort to get more sound, they complain about comfort. Our design of cradling in the bowl of the ear while funneling sound into the ear canal, gives the user both improved sound and comfort, while our gentle Comply foam reduces ambient noise.

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You can learn more about Comply / Hearing Components by reading this recent review here.

MyMac.com rates Whoomp! Earbud Enhancers overall at 3 out of 5.