I replaced my original Hitachi head with this, mostly for cleaning reasons - now, it adds several new dimensions to the already thriving sex life between me and my Hitachi! I use it mainly on my clit (to fabulous effect) and the surrounding areas, though the vibrations feel great on most other parts of the body (it is technically a massager, after all).
I use this toy mainly for masturbation, though it can be a great enhancer for partner sex as well! The head may be too bulky for use during face-to-face intercourse, but could be an amazing addition to doggie-style/rear-entry positions. To the ladies who love ladies: try sandwiching it between the two of you and playing with both textured sides at once! Really, anywhere you can put it will feel great, regardless of who you're having sex with or how - just be creative, and take advantage of the interesting new textures this cap provides!

This silicone cap has a smooth texture (but only a little smoother than the lightly textured original Hitachi head) and grips the skin slightly, if it is dry. Due to this, I prefer the original (rubber?) head for massage on my muscles, as it slides across the skin easily. This cap grips the skin and doesn't slide across smoothly.
That said, on a wet vulva it's heavenly. The silicone is firm but yielding, smooth (my favorite), and wonderfully easy to clean (one of this toy's biggest plusses)! That said, this thing definitely collects lint (with such enthusiasm!) and needs cleaning often.

Here's the fun part! The three horizontal ridges, plus the single vertical ridge with 'tongue' each add an interesting new element into the Hitachi's vibration. I really like both of these features, as well as the smooth parts between, and interchange between them for variation when I do solo play.
The three ridges are subtle, though they definitely intensify the vibrations into smaller points of contact. It feels great to run the ridges over my vulva, and when they hit my clit... well! I tried using it sideways, so that the ridges ran vertically, but I didn't enjoy the sensation much more, and it was an awkward position to maintain. I like this feature for when I want slightly more intense stimulation than the smooth areas provide, but not as single-focused sensation as the single ridge will give.
The single ridge and 'tongue' are good for a more intensely focused vibration. After a bit of build-up, I like to nestle this bit in between my lips, right on my clit, for the intense sensations I need to orgasm. I don't prefer the protruding 'tongue' bit (it's really not like a tongue at all) - it's too sharp a focus on my clit, the stimulation is too much for me. It's nice when it's just above my clit, and the more gently rounded bit delivers the direct vibes. It's also fun to rock your pelvis back and forth on this bit, especially with lube!

I really like this toy, since it has the ability to transform each of the Hitachi's dual-speeds into 3 separate intensities! (not to mention all the possible pressure variations) I only use the lower speed, and so with the three main textures of this cap (smooth medians, triple ridge, and single ridge) I can intensify / change sensations within one speed setting. Now that's neat!
Also, practically speaking, it's much easier to put on and take off this cap that is was to remove the original Hitachi head. It just slides gently on and off, yet stays put during use.

Hooray for silicone! I'm always so pleased to be able to thoroughly clean my toys, aren't you? The ease of cleaning was the main reason I purchased this cap - the subtle texture of the original Hitachi collected fluids, was hard to clean, plus I'm still not sure what it was made of (yikes!).
As this is silicone, it is incredibly safe to use and is easily cleaned: just wash in warm soapy water or boil for several minutes or run through the top rack of your dishwasher. Done.
Now, the catch... it's hard to keep clean. It collects lint more than any other silicone toy I own. Between washing and use (even if it's only a 3 minute delay!) this thing collects tiny bits of fiber that float on the air. I would recommend keeping some wet-wipes or toy-cleaner handy for immediately before use, otherwise you may have to take it to the sink every time you want to use it. Kind of a mood-killer...
The answer may be to put the cap back in its original packaging (a nice plastic cylinder from Vixens) right after cleaning - then, when you take it out right before use, it won't have had the time to collect floating dust. I haven't tried this idea yet, but I'll let y'all know how it goes!

Just to make a point:
While 'researching' for this review, I got intensely distracted by how much I enjoy this toy! In fact, I've been at this review for hours and it may be a while still... you know, I need extra time to, uh, clarify my thoughts. Yep.